Thread: Speed vs fuel consumption

[QUOTE=trout1105;2941850]

Originally Posted by 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.[/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

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

[QUOTE=Tombie;2941893]

Originally Posted by trout1105

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

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.

Originally Posted by 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.
Attachment 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😜

[QUOTE=trout1105;2941906]

Originally Posted by Tombie

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.

Originally Posted by JDNSW

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.

[QUOTE=Tombie;2941959]

Originally Posted by trout1105

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.

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.

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
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