Speed vs fuel consumption

[JDNSW]

On my recent trip to Perth and back I have kept track of fuel economy. In addition, I was in a hurry to get there for a conference, which meant that on the trip west, we travelled at a target speed of 110kph where this was legal (probably mostly 105-110), but on the way back there was no such imperative, and except for the second last day (I had ideas of making it home that day, but got too tired), I worked on a target speed of 100kph.

Having topped up the tank yesterday (so the distance includes an extra trip to town), I now have the figures, which give us an idea of the effects of that extra 10kph. Travelling 3976km to the outskirts of Perth averaged 11.90 l/100km, and on the return trip, 4152km, including a bit of driving in Perth, averaged 10.80 l/100km.

So it seems that increasing the driving speed by 10% means an increase in consumption of a bit over 10%.

This is with the Isuzu engine, but since the extra drag is almost entirely from the 'aerodynamics' of the vehicle, I expect the results are applicable directly to any 110 wagon, and probably indicative for any 90/110/Defender. Diesel engines operating in the part of the power band would not have any appreciable difference in efficiency for a 10% change in speed, so I expect the relative results to apply equally to any engine - Isuzu, Tdi, Td5, Tdci.

Petrol engines, especially older ones, may have efficiency that varies more, which may make the results less applicable, and more streamlined and lower frontal area vehicles may have a smaller effect.

[trout1105]

Try a test run at 85-90kph instead of 100-110kph, You will be amazed at the further reductoin in fuel consumption especially in "Boxey" non areodynamic 4WD's also If you are towing a van or a boat with any sort of cab on it the reduction in speed will save you plenty at the bowser and your engine will Love you for it[thumbsupbig]

[scarry]

We did the same test,at those two speeds with the D4.loaded,over 600Km.Actual speed,as per GPS,100,and 110km/hr.

It used just over 10% more fuel at the higher speed.

Ambient was 40 degrees,and we had one of those point and shoot thermometers with us.

Hub and tyre temperatures also increased between 10 to 15 degrees at the higher speed.

[vnx205]

When I drove to Perth in May, I had a fairly strong headwind most days across the Nullarbor and used over 12 litres/100km most days.

On the return trip, I had a few days of tailwinds and averaged less than 11 litres/100km.

While my ground speed was about the same, my airspeed was probably different by at least 30 km/h.
154523
My setup was even less aerodynamic than normal, so wind resistance would have been an even larger issue.

[headdamage]

Funny but I just did a 500km trip each way. 110 kph on the way up and 100 kph on the way back, total 1000 km. Fuel consumption on the way up was 13.33 L/100 km, the way back was 12.86 L/100 km. Unfortunately this was not with one of my Rovers, it was my 2006 2500HD Duramax.

In the past I have noticed that my 200tdi RRC can do much better at 90 kph than 110 kph. On one 800 km run I made a point to try and keep it closer to 90 kph and got 7.5 L/100 km, it normally gets around 9 ish on the hwy.

With my 90 I've only just fitted a 200tdi into it and did a 5 day 900 km combined hwy and off road camping trip. I had a problem with it and could only get 8 psi boost (I've since fixed this and run around 15 psi with much better pickup). With the low boost I got 11.5 L/100 km, I'm expecting this to improve now it is running better.

[prelude]

iirc drag is not a linear function so 10% difference in speed is not perse 10% difference in fuel consumption but it has been a loooong time :)

What I want to point out though (apart from being obvious perhaps) is that in my experience an engine (diesel or petrol) uses a "fixed" amount of fuel at a minimum. The engine has a certain volume and inside that volume an air/fuel mixture needs to be present. When increasing speed the engine turns over more quickly and thus more of the engine capacity get's pumped through from intake to exhaust in the same amount of time. That fact in and of itself will cause you to burn more fuel. So, yes, drag is probably a factor in the increase in fuel consumption, more rpm's will also use more fuel.

Try driving a large stretch in 4th gear in stead of 5th and see what that does, at the same speed.

Cheers,
-P

[JDNSW]

iirc drag is not a linear function so 10% difference in speed is not perse 10% difference in fuel consumption but it has been a loooong time :)

What I want to point out though (apart from being obvious perhaps) is that in my experience an engine (diesel or petrol) uses a "fixed" amount of fuel at a minimum. The engine has a certain volume and inside that volume an air/fuel mixture needs to be present. When increasing speed the engine turns over more quickly and thus more of the engine capacity get's pumped through from intake to exhaust in the same amount of time. That fact in and of itself will cause you to burn more fuel. So, yes, drag is probably a factor in the increase in fuel consumption, more rpm's will also use more fuel.

Try driving a large stretch in 4th gear in stead of 5th and see what that does, at the same speed.

Cheers,
-P

Aerodynamic drag increases as the square of the speed, so fuel consumption increases as the square of the speed (once above the speed where aerodynamic drag represents most of the drag, typically above about 60kph). But this increase is relative to time, and we are interested in fuel consumption per kilometre. Since the time taken is inversely proportional to the speed, the fuel consumption per kilometre is linearly proportional to speed.

Specific fuel consumption of an engine is the fuel per unit time per unit power, and for diesel engines or indeed for modern petrol engines is fairly constant in the range of powers typically used on the highway, and also fairly constant for different rpm (all within the sort of speeds we are talking about).

While higher rpm typically involves increased losses from 'pumping' and aerodynamic losses within the engine, since the engine is producing more power, the percentage loss is usually much the same, or even lower - there are fixed losses such as cooling, alternator, power steering pump, aircon, vacuum pump, which represent a higher percentage of power and hence fuel at low power settings. All of these are pretty negligible when talking about 10% differences in speed.

If you start driving very slowly, where aerodynamic drag becomes insignificant, these become much more important, as most of the other drag forces are proportional to speed not the square of the speed.

Then there are complications, such as variations in combustion efficiency with engine speed and throttle opening, but it is difficult to generalise about these.

[ATH]

I do like all that guff from JDNSW. Didn't understand a word of it but it was a great read. :) Keep the right foot up a bit and it'll make a difference whether towing or just driving somewhere, anywhere at any time.
Gawd! So many getting so uptight and technical about a litre or two. I think I might have to have a drink or two to calm down.
AlanH.

[DiscoMick]

Road conditions can also make a significant difference.
Towing our Guardian camper, our Defender used 11.5 l/110 cruising up the Pacific Highway, mostly motorway, from Sydney to Brisbane in the last school holidays.
Today we did about 600ks from Gayndah to Emerald, much of it climbing and falling, and used 12.5 l/100 Ks. Same vehicle and camper, similar speeds, different road conditions.
I notice the grey nomads seem to tow their vans at about 90-95 to save fuel.

[clive22]

Power required to increase speed is cubed to the speed. So an increase in speed of 10 percent requires over 30 per cent more power. 1.1^3. Dividing distance traveled the fuel by 10 requires 21 per cent. 1.1^2=1.21 or 21%

Power required is squared again over drag over the same time as the work over that given distance is doubled.

Or fuel consumption increase at the square of the speed when you divide distance through again when assessing fuel use over a given distance.

Engines are most efficient at peak torque which is on one measure optimal gas flow. This will effect your comparison as engine efficiency declines either side of peak torque.
Generally the lowest consumption will be found at peak torque in the highest gear. Of course weight, tail winds, tyre drag, part throttle efficiencies all effect this so it will be difficult to arrive at a perfect figure.
In my observation most vehicles seem to give their best at 80 to 90 km per hour
Clive

[JDNSW]

Yes, as a general rule, engines are most efficient at peak torque - and an estimate of how sharp this peak efficiency is is given by how sharp the torque peak is. Specific fuel consumption curves are rarely published for car engines. Even where they are published, note that they are for specific fuel consumption at full throttle. How often is any practical vehicle driven at full throttle at maximum torque rpm? (Some trucks are, but all current cars and light vehicles have far too much power for this to be practical.)

However, this peak in energy efficiency has to be seen against a drag curve that has no peak - it just keeps going up.

Fuel consumption also depends on the road - hills, curves that need slowing for, traffic that requires changes in speed.

But also very important is driving style. And, apart from speed, the main difference is in use of brakes. Brakes convert speed into heat, representing directly, waste of fuel. Economical driving means driving as if the brakes do not work - look ahead, anticipate the need to slow, and do so without braking. Also, allowing the vehicle to slow approaching a crest, so you can safely and legally allow it to accelerate down the other side of the hill, again, avoiding braking, saves fuel - where this is practical.

The reason this trip to Perth was a good test of the effects of speed is that most of the distance is flat, straight and little traffic. And, importantly, long enough that errors in fuel quantity when filling up (did I fill it to exactly the same level?) are insignificant. A possible confounding factor could be a prevailing wind, but my observation was that wind was either light or a crosswind on both legs.

And then there are variables such as tyre pressure - if the tyre temperature rises significantly, you are wasting fuel!

Driving with windows open versus using the aircon - depends on the vehicle; the more streamlined the bigger the effect of open windows, but probably not a significant effect on a brick like a Defender! On this trip, no aircon, no windows open, vents occasionally open - but mostly wishing the heater worked better!

[gruntfuttock]

But also very important is driving style. And, apart from speed, the main difference is in use of brakes. Brakes convert speed into heat, representing directly, waste of fuel. Economical driving means driving as if the brakes do not work - look ahead, anticipate the need to slow, and do so without braking. Also, allowing the vehicle to slow approaching a crest, so you can safely and legally allow it to accelerate down the other side of the hill, again, avoiding braking, saves fuel - where this is practical.




There used to be an old fella around the area who drove on a similar principle many years ago. He drove an old Ute and costed down hill only self starting it on the up hill section once it dropped to below 10 mph and cut the ignition just before he got to the top. He also had in his shed a 44 gallon drum of “Plume Benzene” still sealed just in case fuel run out! (And you couldn’t tell him it was no good either) I think he also had his first pay packet as well!! If you got stuck behind him, and many people did, you would know it. :bat::bat:

[Tote]

I did a job at Wilcannia many years ago commissioning a small offset printer, I had an offsider with me and we had identical Falcon panel vans. On the return journey it was dark and we decided that I would be in front as I had a set of 130 watt aircraft landing lights on my van. We filled up at Wilcannia and refilled the next morning in Cobar. My vehicle used about 10 litres more fuel over the 260KM run due to the extra load from the alternator.

Regards,
Tote

[scarry]

I did a job at Wilcannia many years ago commissioning a small offset printer, I had an offsider with me and we had identical Falcon panel vans. On the return journey it was dark and we decided that I would be in front as I had a set of 130 watt aircraft landing lights on my van. We filled up at Wilcannia and refilled the next morning in Cobar. My vehicle used about 10 litres more fuel over the 260KM run due to the extra load from the alternator.

Regards,
Tote

i worked for a company that also ran a fleet of Falcon vans,in the 80's.
We also did a lot of country miles.

Surprisingly,we found that two vans,exactly the same spec,same month built,on a run,one following the other would have quite large fuel usage differences.

Actually,i remember driving to Mt Isa and back a few times,with two XF vans,before they went to unleaded,built same month,same weight,same empty roof racks,same bull bars,both only had around 6000k on them,identical.But one used around 10% more fuel than the other.

We also found that some,same spec,would be faster than the other,by quite a margin.

[Tote]

From Cobar out in the morning they used within a litre of each other..........probably should have put that in the story. You're right about the variation though I had about 5 different XF vans and no two were remotely similar. Different diffs, 3 speed vs 5 speed, unleaded vs leaded etc. The best one I had was the last of the leaded ones, a three speed with a limited slip 2.92 diff. The two we took out to Wilcannia were both 5 speeds, I can't remember if they were 3.3s or 4.1s.

154574

154575

Regards,
Tote

[scarry]

From Cobar out in the morning they used within a litre of each other..........probably should have put that in the story. You're right about the variation though I had about 5 different XF vans and no two were remotely similar. Different diffs, 3 speed vs 5 speed, unleaded vs leaded etc. The best one I had was the last of the leaded ones, a three speed with a limited slip 2.92 diff. The two we took out to Wilcannia were both 5 speeds, I can't remember if they were 3.3s or 4.1s.

154574

154575

Regards,
Tote

Ours were all 4.1,auto,the XD was lower reving,Ford changed the diff ratio for the XE onwards,which made it a much nicer vehicle to drive,and more economical.
I can't remember the ratios,but the auto wagons we had were higher again,maybe 2.77?
We had steel bars,then alloy XF onwards,but not full bumper as in your pic.
Unfortunately i have no good pics of ours.

The unleaded XF didn't go as well as the leaded model,from memory.
My favourite one was an XE,it was such an improvement over the XD.

Myself and some of the other guys,used to move the cargo barrier back about 450mm, then screw the baby seat side on,just in front of the cargo barrier,if you know what i mean.Between the cargo barrier and the rear of the front seat.It worked well.
I also had the Engle wired up with a second battery in the rear,for camping,and holidays,and long work trips.

Those were the days!

Thinking more about it,i think the XD diff ratios were 3.36,XE onwards was 2.92.
Bloody fuel gauges used to play up,i always had a jerry in the back on trips.

[AK83]

......
In my observation most vehicles seem to give their best at 80 to 90 km per hour
Clive

agree [thumbsupbig]
That's basically what I do.
My drives usually consist of overnighting it to my destination to be there at a sort of specific time.
So I have plenty of time(more accurately I give myself plenty of time), and just tootle along which saves tons of fuel.
Can get caught out sometimes, as overnighting it, usually means fuel is hard to come by out bush.

Had a Rodeo 3.2 V6(99 model) which was duel fuel.
On a trip up to Wilpena-Brachina Gorge, I assumed that fuel on the Barrier Hwy should be available 24hrs. Seeing that Burra was a large town, I planned for this to be a spot to fill up at.
Wrong! .. still had to get to Pt Augusta and then on to Kanyaka ruins to get there for sunrise(my destination), and hardly any petrol, and now low on LPG.
Had no option other than to slow down even more than my usual 80-ish k/h.
So sat on 60 for a long while, and even down to 40 for a good bit of it too. stayed in 5th to keep revs low too, where possible.
Rodeos' usual fuel consumption was in the 5 lt/klm on gas, maybe 5.5 ... or on a very good day near on 6km/lt .. motor was old and totally buggered.
From Renmark to Pt Augusta .. took 5.75hrs all up, 3 hrs from Burra(200-ish klms) but it did achieve 6.7 km/lt on LPG that drive.

Other fuel economy run of note, in my old (clapped out too) RRC, I had a similar situation where I was coming back(home) from around Lk Eyre ish area but decided to come home via Broken Hill.
RRC's usual fuel economy was in the 6 km/lt range, a good day used to be 6.5km/lt. Sat on about 70k/h along (primarily) gravel roads(nice and smooth tho) and got to BH 640kms later giving 8km/lt, just under, as it filled with 81lt(on an 80lt tank :D)

As said, I just give myself plenty of time to get there, and sit cruisily at what I think is a good balance between fuel usage and time driving .. I reckon 80-90 k/h is a good balance.

[Tombie]

You guys have too much time on your hands [emoji3]

I love driving, my longest stints behind the wheel have been Perth-Adelaide non-stop (fuel, Food and comfort stops only) and Adelaide to Brisbane about 4 times the same way.

However, the less time I need to take doing so the better.

If that means instead of $14.00 per 100km of fuel I now burn $17.00 per 100km then so be it. Even an extra $200 in fuel is worth far less than my time.

To each their own; just try not to hold up too many people who are trying to reduce their time on the road.

[trout1105]

You guys have too much time on your hands [emoji3]

I love driving, my longest stints behind the wheel have been Perth-Adelaide non-stop (fuel, Food and comfort stops only) and Adelaide to Brisbane about 4 times the same way.

However, the less time I need to take doing so the better.

If that means instead of $14.00 per 100km of fuel I now burn $17.00 per 100km then so be it. Even an extra $200 in fuel is worth far less than my time.

To each their own; just try not to hold up too many people who are trying to reduce their time on the road.

When I am not towing I tend to sit on the speed limit because the fuel/wear and tear savings are minimal.
However when towing I tend to sit on 85/90kph Not only to save fuel but to minimise the wear and tear on the 4WD and trailer, It is also a much safer option.
Many haulage companies now have their semi's speed rated between 80 or 90kph and they wouldn't be doing this in order to pay more wages to their drivers[bigwhistle]

[AK83]

.....

If that means instead of $14.00 per 100km of fuel I now burn $17.00 per 100km then so be it. Even an extra $200 in fuel is worth far less than my time.

To each their own; just try not to hold up too many people who are trying to reduce their time on the road.

If you read my reply ... it's not just about the cost.
But cost does come into it too .. as well as stressing the vehicle out(could be why my clapped out motors usually get me beyond 600K klms too!)
But sometimes it's about the amount of fuel you have access too(at any given time in the drive).

Longest non stop stint(s) I've done have been Melb - Lk Eyre and back(other than food and fuel) and Melb - Eyres Rock and back, with a 2 hr sleep at the village at Uluru, coz I got there 2 hrs earlier than I expected.

But even as a courier, where time was supposedly the primary concern for the customer.
Was a express courier, not the DHL/TNT set run kind .. grab a job and usually go with it type of service.
I used to rush around like a typical motorbike courier thinking that quicker was better .. more jobs done, more $ in pocket.
Never really happened tho, and I can say, not blowing my own horn here, but I was very good at it too .. but I realised one day .. too quick and all that happened was that you missed jobs along the way because you were past the next job booking due to being too quick.
So slowing down(as the courier) eventually got me more $s as I'd pace myself a little more, just anticipate light sequences and traffic better .. less wear and tear, less fuel, less $s cost(for me)!

One of my major gripes with traffic (in and around town) was that everyone seems to think if you maintain the speed limit you'll get there faster, which on the whole was completely wrong.
Slowing down and anticipating the actual average speed of the traffic flow got you there at the same time.
When I was driving trucks fro Woolies(recently, just got redundant) had to deal with the stupid Westgate Fwy every morning and the lunacy of this mad rush to go nowhere fast.
Half the cars in Vic trying to get into the city(and some beyond) .. for those that know it understand the stupidity of it .. plus the perpetual roadworks on it for the past 20+ years.
It's 4 lanes on the whole, and I'd just get into the lane I needed and sat at about 30-ish k/h .. no matter what everyone else was trying to do.
it was obvious that on a good day it's natural flow was about 30-40 k/h and everyone around would hammer to 60k/h then slam brakes on .. hard on the gas to 60 again and hard on the brakes again.
I'd just trundle along very rarely using brakes, and my lane would have a few hundred meters of clear road ahead of me due to my 'slowing down' .. not maintaining the same idiot driving style that all the other sheep would tray too.
Of course my lane being clear WAS the fast lane now, and every fool and their mate would cut into my lane(assuming this was the fast lane).
Always got to the split, where most traffic would veer off for the city and me for the Eastern suburbs at the same time as the idiots thinking that my empty lane was the supposed fast lane.

As for holding anyone up .. where possible possibly, but that was mainly due to anticipation that going faster wouldn't achieve anything(eg. coming to a hairpin, or slowdown of whatever other type)
But as my drives(to Lk Eyre, or Wilpena .. time off type drives) were primarily overnighters and the only traffic to speak of were trucks, I was always mindful of them coming up behind and would adjust to now slow them down in any way.
Otherwise just maintain my speed.
eg. if on a long straight stretch where passing was easy, just hold speed, and trucks would have the momentum to pass. If there was a bend coming up, I'd indicate for him to pass, and I'd slow down even more so to get him back on the right side again quicker.
If there was no other option tho .. I'd run at 100 if I had too, to not slow them down and then slow down at the next opportunity for them to pass, and I'd be back at my speed.

Occasionally I'd come across a bloke cruising at say 90, which is about my cruising speed, and I'd tuck up behind using the slip stream too.
Ceduna to the border village doing this one time, truck man was unfussed with me behind him.

It's just common sense. You don't need to speed to get somewhere on time .. just give yourself time to do so .. and try to not bother anyone.

And on the topic of the cost: that $200 is 50% of my last trip out to Cobar-Broken Hill and back. If I saved that kind of money every drive(time off drive, not driving for work or in general) .. I'd take it any day of the week.
Gets me another 50% further, longer time away, and or more money spent on other stuff(food, maps, entry fees .. whatever).

And as Trout said, Linfox is pushing their drivers to sit on about 90 on the local freeways around here. Make zero difference to time taken in terms of the completed job, less stress on the driver and truck .. etc.

[clive22]

The most efficient speed of your vehicle is an interesting parameter to establish and know. The instant fuel figures on modern cars help heaps with this.
Whilst I normally travel on the speed limit, I for one normally try to drive using conserve momentum using the minimum require throttle to maintain the desired speed.

Its not just frugal's who do this, this conservation of momentum is at the core of all good driving be it heavy haulage, F1, MotoGP, commuting, highway, chauffeuring


Its not about just stomping of the go pedal it about using energy (aka momentum) efficiently. Be it making correct choices about accelerations, hills, corners all whilst considering what is ahead.

Pleasure is not the right word but there is a satisfaction in driving it right.

Clive

[cripesamighty]

It can get even more nitpicky in city driving. UPS in the USA, has programmed the GPS units in their vehicles to mostly turn right and rarely left when city driving. From memory it was because going around the block is statistically quicker than trying to turn against the traffic. I'm sure it will be online somewhere.

[Homestar]

Only fuel test versus speed I did in the 101 was a 300KM trip - traveled there at a sedate and easy 85KPM and averaged 21's. On the way back, pulled the overdrive in and cruised at 100 to 105KPH - averaged 28's doing that... [biggrin]

Did someone say house brick???

[DiscoClax]

Last time we were up near Alice I checked the numbers in the D1 at 100 and again at 130 a few times. Fully laden with a bunch of stuff on the roof. 55% throttle at 130 and around 25% at 100. Only wound it up to 130 to say I'd done it and to see how she'd go really. Highlights the non linear relationship pretty well though. The fuel burn at 130 would have been horrendous. Certainly the wind noise was...

[Eevo]

It can get even more nitpicky in city driving. UPS in the USA, has programmed the GPS units in their vehicles to mostly turn right and rarely left when city driving. From memory it was because going around the block is statistically quicker than trying to turn against the traffic. I'm sure it will be online somewhere.

ive seen this before. tried it a few times too, definitely true.

[Tombie]

It can get even more nitpicky in city driving. UPS in the USA, has programmed the GPS units in their vehicles to mostly turn right and rarely left when city driving. From memory it was because going around the block is statistically quicker than trying to turn against the traffic. I'm sure it will be online somewhere.

I use a similar strategy in the cities, make a quick left to flow with traffic and then a U-turn on the next traffic island to flip the other direction if required.

Mind you, I have a car at the house in Hua-Hin, Thailand and if you’re comfortable driving there you can do anything! Over there it’s guidelines rather than rules [emoji38]

[Pedro_The_Swift]

I wouldnt have included Moto Gp in that list,, unless you are very old! These days its how quickly you can get the bike upright to accelerate,, hard brakes,turn and shoot, not much Hailwooding done... [biggrin]

[rick130]

I wouldnt have included Moto Gp in that list,, unless you are very old! These days its how quickly you can get the bike upright to accelerate,, hard brakes,turn and shoot, not much Hailwooding done... [biggrin]Yamaha's are still all about corner speed and managing edge grip. Quatararo seems to be mastering that quite well while still 500rpm down on the factory bikes. [emoji16]

[Barraman]

I've just notched up 150,000 km on my 2015 D4 TDV6, about half of that towing a 2,500 kg boat. I drive at pretty much the same speed with or without the boat.

I average about 10 L/100 km without the boat and 17.5 L/km with the boat on.

[trout1105]

[QUOTE=Barraman;2941843]I've just notched up 150,000 km on my 2015 D4 TDV6, about half of that towing a 2,500 kg boat. I drive at pretty much the same speed with or without the boat.

I average about 10 L/100 km without the boat and 17.5 L/km with the boat on.[/QUOTE

Try slowing down when towing the boat, You will be amazed how much better the fuel consumption will be and your D4 will Love you for it[thumbsupbig]

[Blackrex]

It can get even more nitpicky in city driving. UPS in the USA, has programmed the GPS units in their vehicles to mostly turn right and rarely left when city driving. From memory it was because going around the block is statistically quicker than trying to turn against the traffic. I'm sure it will be online somewhere.

Mythbusters Mini Myth.


https://www.youtube.com/watch?v=gMRp4RqEsHk

[Tombie]

[QUOTE=Barraman;2941843]I've just notched up 150,000 km on my 2015 D4 TDV6, about half of that towing a 2,500 kg boat. I drive at pretty much the same speed with or without the boat.

I average about 10 L/100 km without the boat and 17.5 L/km with the boat on.[/QUOTE

Try slowing down when towing the boat, You will be amazed how much better the fuel consumption will be and your D4 will Love you for it[thumbsupbig]

Don’t know where you get the idea the vehicle will “love” you for going slower. The wallet might but at 100km/h the engine is ticking over, it’s in one of the top gear ratios and is ticking along.

Drop a gear or 2 to sit at 80-85km/h and the engine is still working the same rpm. Yes it will be working a bit less hp/torque but it will be doing it for a significantly longer portion of time.

So if the wallet permits get on it [emoji3]

[trout1105]

[QUOTE=trout1105;2941850]

Don’t know where you get the idea the vehicle will “love” you for going slower. The wallet might but at 100km/h the engine is ticking over, it’s in one of the top gear ratios and is ticking along.

Drop a gear or 2 to sit at 80-85km/h and the engine is still working the same rpm. Yes it will be working a bit less hp/torque but it will be doing it for a significantly longer portion of time.

So if the wallet permits get on it [emoji3]

Where did you get the insane idea that you have to drop down a gear or two to travel a bit slower, All you have to do is ease off on the throttle a bit.

[1103.9TDI]

When I drove to Perth in May, I had a fairly strong headwind most days across the Nullarbor and used over 12 litres/100km most days.

On the return trip, I had a few days of tailwinds and averaged less than 11 litres/100km.

While my ground speed was about the same, my airspeed was probably different by at least 30 km/h.
154523
My setup was even less aerodynamic than normal, so wind resistance would have been an even larger issue.

Haha, you might be able to throttle back a bit with a tailwind, but I can assure you, in a Landrover, your groundspeed is going to be the same as your airspeed, unless you are in the air, in which case it’ll probably be slower depending on how much gravity is involved😜

[Tombie]

[QUOTE=Tombie;2941893]

Where did you get the insane idea that you have to drop down a gear or two to travel a bit slower, All you have to do is ease off on the throttle a bit.

An 8 speed auto will shift to maintain torque band and will be in around 6th.

Not insane; experience.

Same with lugging a 4.5V8 cruiser at lower rpm in top gear is a recipe for higher EGTs and destruction of the gearbox. Experience.

[realchris]

Aerodynamic drag increases as the square of the speed, so fuel consumption increases as the square of the speed (once above the speed where aerodynamic drag represents most of the drag, typically above about 60kph). But this increase is relative to time, and we are interested in fuel consumption per kilometre. Since the time taken is inversely proportional to the speed, the fuel consumption per kilometre is linearly proportional to speed.

Specific fuel consumption of an engine is the fuel per unit time per unit power, and for diesel engines or indeed for modern petrol engines is fairly constant in the range of powers typically used on the highway, and also fairly constant for different rpm (all within the sort of speeds we are talking about).

While higher rpm typically involves increased losses from 'pumping' and aerodynamic losses within the engine, since the engine is producing more power, the percentage loss is usually much the same, or even lower - there are fixed losses such as cooling, alternator, power steering pump, aircon, vacuum pump, which represent a higher percentage of power and hence fuel at low power settings. All of these are pretty negligible when talking about 10% differences in speed.

If you start driving very slowly, where aerodynamic drag becomes insignificant, these become much more important, as most of the other drag forces are proportional to speed not the square of the speed.

Then there are complications, such as variations in combustion efficiency with engine speed and throttle opening, but it is difficult to generalise about these.The air resistance, i.e. force is proportional to speed squared. Work, or energy, for a set distance is force times distance. So the energy per unit distance is proportional to the vehicle speed squared. The energy required should be proportional to the fuel used.

So for the air resistance component at least, the fuel usage per 100km is proportional to the square of the vehicle speed (assuming equal engine efficiency at the different speeds).

Engine power is different. Power is equal to speed times force. So the engine power is proportional to the cube of the vehicle speed (which another member has mentioned). Which is why you need such massive engines for high speed racing and land speed records - even a moderate increase in power only increases the top speed by a little bit.

[trout1105]

[QUOTE=trout1105;2941906]

An 8 speed auto will shift to maintain torque band and will be in around 6th.

Not insane; experience.

Same with lugging a 4.5V8 cruiser at lower rpm in top gear is a recipe for higher EGTs and destruction of the gearbox. Experience.

Are you saying that an 8 speed auto is Incapable of travelling at 90 KPH in top gear??
I haven't owned an 8 speed auto But if this is the case then I wouldn't have a bar of one
I have done Thousands and thousands of K's towing at 85-90kph in the 79 series and the gearbox and engine are just fine thank you even though it is manly in top gear for for those K's.

[Tombie]

An 8 speed will find the optimal gear for torque based on speed and load. So towing heavy at 80-85 will have it heading down the box.

Could be worse - have a read of the Toyo handbook - tow in 4th! No wonder there are so many slow cruisers out there! Add to that the Toyo manual box is made of some form of plasticised Custard and towing in OD on one of them is asking for a rebuild.

Try it with areas of lots and lots of hills - Eg Eastern seaboard states. The stats back it. Even the maker recommends against heavy towing in top.

And come talk to me about the 79 after around 150-200k. You may be a lucky one. There’s 2 blokes here that have had nothing but problems from new and Toyota won’t do a damn thing about them.

[trout1105]

And come talk to me about the 79 after around 150-200k. You may be a lucky one. There’s 2 blokes here that have had nothing but problems from new and Toyota won’t do a damn thing about them.

My 79 series has done Well over 300,000K's [biggrin]
Yes in Hilly areas I drop down a gear or two to get up the hills IF necessary But on mild hills there is NO need to downshift at all

[Tombie]

My 79 series has done Well over 300,000K's [biggrin]
Yes in Hilly areas I drop down a gear or two to get up the hills IF necessary But on mild hills there is NO need to downshift at all

You’re a lucky guy then.

I’m talking experience with 30+ of them.
Half had had full fuel systems replaced and only about 8 have the original gearbox in them.

These vehicles can tow daily up to around 2t on formed roads.

Most of them are like Grandpa‘a Axe... so many new parts they really are the original unit anymore.

Look after yours sounds like you got a good one.

[Eevo]

Most of them are like Grandpa‘a Axe


i feel like my d2 is like that.

[Tombie]

i feel like my d2 is like that.

Aside from springs, shocks, radiator, uprated P38 trans cooler, some gaskets and a front driveshaft our D2 is all factory build components.

[trout1105]

Aside from springs, shocks, radiator, uprated P38 trans cooler, some gaskets and a front driveshaft our D2 is all factory build components.

Aside from the steel rear bumper on my D2a it is also comprised of factory built parts, The trouble is that Most of those parts have had to be replaced or rebuilt.
The D3's and D4's will be No different as they age.

[Tombie]

Aside from the steel rear bumper on my D2a it is also comprised of factory built parts, The trouble is that Most of those parts have had to be replaced or rebuilt.
The D3's and D4's will be No different as they age.

Sorry I should have been clearer.
What I mean is the parts fitted on the assembly line.

Suspension, gaskets etc are consumable so I don’t mind that.

[Eevo]

Aside from springs, shocks, radiator, uprated P38 trans cooler, some gaskets and a front driveshaft our D2 is all factory build components.

original things left

gearbox
rear diff
chassis/body
rims

[Ancient Mariner]

Y62 Y would you
https://youtu.be/eL1eJQNn2i8 More noise and rattles more than my Defender [bighmmm]


AM

[loanrangie]

Nothing ****s me more than people not doing the speed limit when the conditions allow, if you can't/won't do it then keep off the ****ing road.

[Thomas27488]

Nothing ****s me more than people not doing the speed limit when the conditions allow, if you can't/won't do it then keep off the ****ing road.

You do realise that the speed limit signs are a recommendation as well as a maximum? You're allowed to drive slower then the speed limit, you just cant drive abnormally slow to obstruct traffic (Example used is, 20kmh in an 80kmh zone).

No need to get agro about it.

[shack]

You do realise that the speed limit signs are a recommendation as well as a maximum? You're allowed to drive slower then the speed limit, you just cant drive abnormally slow to obstruct traffic (Example used is, 20kmh in an 80kmh zone).

No need to get agro about it.Ha! I actually know of someone who got pulled over on national highway 1 for loitering. It needed to be done.

[trout1105]

Ha! I actually know of someone who got pulled over on national highway 1 for loitering. It needed to be done.

Compared to the amount of people busted for speeding on the roads the odd "loiterer" is irrelevant.[bigwhistle]

[Tombie]

Compared to the amount of people busted for speeding on the roads the odd "loiterer" is irrelevant.[bigwhistle]

Hardly, again let’s forget the marketing hype.

Of course speed kills, but safer vehicles on better roads and we’re now going slower...

There are areas where faster speeds would be far safer, reduce road incidents and road rage...

[clive22]

Absolutely some roads are designed to for safe traveling at up to 130km/h. The Hume for example.
Sitting on a 110 km/h in a modern vehicle is really a waste of the road and the vehicle's capabilities.
I can't see why they are not raised. It reduces travel time, limiting exposure on the road, a key component of mitigating risk
A lot of Countries have a 'typical' driving speeds of 130km/h on two lane two way divided roads.
None of this clumping rubbish with everyone driving bunched up, 10-20m apart, but too scared to speed.

Clive

[JDNSW]

We seem to have got a little off topic. The experiment I first posted about was looking at the difference in fuel consumption with speed, and confirmed that fuel consumption per kilometre is directly proportional to speed once you are travelling fast enough that most of the resistance is aerodynamic as theory predicts.

Historically this conclusion has been distorted by vehicles having a limited number of gears and having engines with a relatively sharp peak in efficiency, but the demands for fuel efficiency have ensured that the vast majority of vehicles have a set of gear ratios to allow efficient operation at any reasonable speed - and usually have a gearbox that automatically selects the optional ratio.

Note that this experiment was conducted over a long distance (close to 4,000km) of mostly flat going with little traffic and few bends or intersections for most of the distance. Especially where there are hills, traffic or bends and intersections introduced, driving style becomes much more important, especially efforts to keep a fixed speed (usually the speed limit). This means high acceleration if forced to slow, and braking to avoid going over the limit rather than planning ahead by taking your foot off well before arriving at the corner etc.

A bit off topic, note that the speed limit is just that, a limit, not a target, and in NSW at least, there are classes of drivers and vehicles that are mandated to a maximum speed up to 20kph below the posted speed limit (90vs110), so the authorities certainly do not agree that everyone should drive at the same speed - even on major highways with only one lane each way and few overtaking opportunities. You may not like this, or think this is sensible, but this is the law.

(At least NSW dropped the speed limit for trailers over 750kg of 80kph - used to be a nice little earner from interstate drivers)

[Thomas27488]

Good summary John. Thanks for keeping the numbers and figures from your trip. Lots of good info here for anyone wanting to find out about fuel economy!

[djam1]

I find that the discipline of driving at a defined speed eg 90kmh soon fades
If I have 1000 km to drive today then I will go as fast as is legal safe and within the vehicles capability
Fuel economy becomes irrelevant after a few hours

[rocket scientist]

Interesting to compare your figures John.
Just did a trip the other day in the Isuzu County, down to Melbourne and back.
300km of country road and 150km of moderate to heavy traffic:- 45ltrs of fuel. 10ltrs/100km.
I usually sit on about 90kph to maximise economy. I have compared doing the trip at 100kph on the open road and 90kph.- time difference is probably 10 minutes.
I am always impressed with the economy puttering around in the traffic all day.

Pete

[JDNSW]

I do very little in the County in traffic, so have little opportunity to see how I go in those circumstances. Driving in traffic - puttering around - the vehicle will rarely reach speeds where aerodynamics are the main source of losses, so driving style will be the principal difference in results, mainly the amount of braking you do (plus things such as tyre pressure, dragging brakes etc).

[trout1105]

There are areas where faster speeds would be far safer, reduce road incidents and road rage...

Seriusly????

The ONLY time that extra speed (momentum) becomes a "Safety" factor is when driving on soft sand, claypans or damp black soil country so that you don't become bogged in a remote area.
ANY increase in speed will increase the risk of injury or death and this is a proven Fact.
As far as road rage goes, Well people that are prone to road rage should NOT be allowed behind the wheel in the first place.

[AK83]

.....
I usually sit on about 90kph to maximise economy. I have compared doing the trip at 100kph on the open road and 90kph.- time difference is probably 10 minutes.
.....
Pete

That's what I've always found. Trying to go faster to get somewhere quicker usually ends up in frustration and zero nett effect!
I give myself just a few more minutes to get there at the slower pace.

[Ancient Mariner]

My 110 weighs 3500 kg, has the last of the isuzu mechanical injection engines 4.7 .4 cyl slightly bigger than the Toyota v8 but less power and torque.At 100k it is doing 1750 revs On a recent trip
Atherton to Jervois in NT speed 80 to 100 dependant on conditions. 10.5. liters per 100 From Jervois to Birdsville down the Hay river track 750k with detours 16 liters per hundred .From Birdsville towards Windora needing to clear the cobwebs 130 for a while ,didn't want to know[bighmmm] but guess about the same as in the Simson

AM

[Tombie]

Seriusly????

The ONLY time that extra speed (momentum) becomes a "Safety" factor is when driving on soft sand, claypans or damp black soil country so that you don't become bogged in a remote area.
ANY increase in speed will increase the risk of injury or death and this is a proven Fact.
As far as road rage goes, Well people that are prone to road rage should NOT be allowed behind the wheel in the first place.

Yes seriously... there is data to back it up.

Perhaps turn off you bias for a bit and do some research.

Granted driver training in Australia would need a damn good shake up!

Road rage is a catch-all for frustration, aggression and includes the frustration leading to taking increased chances or risks through others actions.

The data supports higher speeds, raised limits, shorter travel times. What it then doesn’t support is revenue.

[Tombie]

Back to fuel economy - D4, full armour, full rack, awning, roof mounted recovery tracks, bullbar etc..

640km round trip Thursday - average speed 112km/h according to trip journey on GPS.

Confirmed fuel consumption (bowser vs dash readout) 10.1L/100km.

Sure I could have saved $8.96 and slowed to 80km/h and then driven for 8 hours (vs 5.7 hours) and been more fatigued - but that just wouldn’t be sensible would it.

[DiscoMick]

Several thousand ks of towing the camper from Brisbane to Lawn Hill, Karumba and Coen, including plenty of unsealed roads, have confirmed our 110 is happiest at 2000-2500 rpm, which means changing down to 5th below about 80 kmh, as 6th is only for highway cruising. Fuel economy is best in that rev range.

[trout1105]

Back to fuel economy - D4, full armour, full rack, awning, roof mounted recovery tracks, bullbar etc..

640km round trip Thursday - average speed 112km/h according to trip journey on GPS.

Confirmed fuel consumption (bowser vs dash readout) 10.1L/100km.

Sure I could have saved $8.96 and slowed to 80km/h and then driven for 8 hours (vs 5.7 hours) and been more fatigued - but that just wouldn’t be sensible would it.

So what were you towing during this trip?
If not towing the extra 20kph makes very little difference in fuel consumption, However If you are towing something substantial that extra 20kph will make a Huge difference in fuel usage.
Also unless you were travelling in the NT your "average" speed of 112kph indicates that you were breaking the law by exceeding the 110kph maximum speed limit[bigwhistle]

[Tombie]

So what were you towing during this trip?
If not towing the extra 20kph makes very little difference in fuel consumption, However If you are towing something substantial that extra 20kph will make a Huge difference in fuel usage.
Also unless you were travelling in the NT your "average" speed of 112kph indicates that you were breaking the law by exceeding the 110kph maximum speed limit[bigwhistle]

Towing the 2.4t Cat will cost an extra $28.00 for the same trip (I’ve done it before in this vehicle and remember the extra fuel burn).

Again - Saving that bit of coin and spending an additional 2 hours behind the wheel is not worth the perceived saving vs the Fatigue.

I always set my Cruise speed ‘optimistic’ which in this case was 116km/h. I never even get a cursory from the HWP as I pass them and I passed about 7 on this run.
Must make me completely evil and irresponsible [emoji48]

Fuel is the cheapest part of motoring, and I’m not giving away life in exchange for economy saving. In fishing terms - 2 hours more on the water vs 2 in the tow vehicle is a no brainer....

[gavinwibrow]

Back to fuel economy - D4, full armour, full rack, awning, roof mounted recovery tracks, bullbar etc..

640km round trip Thursday - average speed 112km/h according to trip journey on GPS.

Confirmed fuel consumption (bowser vs dash readout) 10.1L/100km.

Sure I could have saved $8.96 and slowed to 80km/h and then driven for 8 hours (vs 5.7 hours) and been more fatigued - but that just wouldn’t be sensible would it.

Hi Mike. So main good fuel consumption claims to fame are? I'd assume from previous comments that the Rocket Rod "chip" is one contender, plus being in SA the ground is reasonably flat.
For the following post of $28 for towing the 2.4T cat, what does that translate to in L/100 km?


Incidentally, from my experience, I suspect towing anything over 2/2.5T leads to exponential/geometric progressions in fuel useage with speed.

[Tombie]

With the Cat on the back cruising at posted speed limits yields 13l/100km.

The run south is a series of continually changing ascents and descents.

With the van (around 2t - it’s small) on the back from Melbourne we averaged 13.2l/100km cruising posted limits the entire way.

Sometimes it’s just the aerodynamics of the combination that will do you in.

Interestingly - with the Bimini up on the Cat we do better than with it down - the Bimini deflects the airflow rather letting it vortex around inside the hull inducing drag.


My modifications are:
Decat & custom exhaust.
No EGR
Remap

When on highway towing left at normal height, when vehicle only I lower it on Llams and run -20mm

[Tombie]

https://uploads.tapatalk-cdn.com/20191013/1f51ae7090ef38d0bda62411b1719d3f.jpg

This is the combo.

[prelude]

Sorry, completely OT, is that a ham radio tower in the background there? ;)

[Tombie]

Sorry, completely OT, is that a ham radio tower in the background there? ;)

Regional. Tv antenna.

There’s a nice HAM set up just up the road though.