Speed vs fuel consumption

**JDNSW** · 26th September 2019, 05:26 AM

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** · 26th September 2019, 06:54 AM

Originally Posted by JDNSW

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.

**Tote** · 27th September 2019, 07:01 PM

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** · 27th September 2019, 07:41 PM

Originally Posted by 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

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** · 27th September 2019, 07:47 PM

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.

scan0002.jpg

scan0004.jpg

Regards,
Tote

**scarry** · 28th September 2019, 01:11 PM

Originally Posted by 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.

scan0002.jpg

scan0004.jpg

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** · 1st October 2019, 07:55 AM

Originally Posted by clive22

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

agree

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

)

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** · 1st October 2019, 08:11 AM

You guys have too much time on your hands

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** · 1st October 2019, 09:28 AM

Originally Posted by Tombie

You guys have too much time on your hands

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

**AK83** · 1st October 2019, 10:25 AM

Originally Posted by Tombie

.....

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.

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