What's the most fuel efficient engine speed?

[allgonepetetong]

I was driving home on Sunday morning after stayin with friends, and the petrol guage was a little lwer than I had anticipated. I had 65 miles to drive on the M40 and M25 and the trip computer was showing I had 80 miles range.

This should be fine but I know the trip computer undereads massively for MPG so I thought I would set the cruise at 65 to ensure i could get home without running out of fuel.

This got me thinking, what would be the most fuel economical speed / engine RPM to travel at?

I'm thinking that peak torque would be most efficient, but this it at 4500rpm in my car and in top that's around 90mph. I could cruise in 5th at around 75mph but that's defeating the point as I would be travelling for 8 more minutes and burning fuel all that time.

So, is top gear at peak torque really my most eficient (90mph) cruising speed?

[V88Dicky]

Too many variables to list probably.

Drag co-efficient of the car, gearing, torque, power, atmospheric conditions, tyres, tyre pressures, engine braking and so on.

[RobCrezz]

Peak torque isnt always best for fuel economy.

I would say usually between 50-60mph as beyond that the drag caused by air will cause a bigger drop in mpg.

[Gruber]

allgonepetetong said:

So, is top gear at peak torque really my most eficient (90mph) cruising speed?

ETA: Talking rubbish - deleted...


As already suggested, there are too many variables. But if it is economy cruising you want, I can't see why you'd want to be at peak torque.

Edited by Gruber on Monday 7th November 13:12

[Dracoro]

In general, as low a speed as possible but above the speed where the engine would labour.

As for the peak torque bks, that would mean me driving around at 7500rpm all the time

There two things going on here though, are you asking which is the best for mpg or which is the most efficient (i.e. best performance per unit of fuel used etc.).

[kambites]

It varies, but I think it's almost always between about 30 and 50mph (certainly all the cars that I've ever owned have been in that range). Typically in the highest gear that you aren't labouring the engine in. I suspect very few cars are most economical in top gear.

Engines are most efficient at around peak torque, but that says little about how to get best economy out of a car - engines are not cars and efficiency is not economy.

Edited by kambites on Monday 7th November 13:18

[allgonepetetong]

Dracoro said:

In general, as low a speed as possible but above the speed where the engine would labour.

As for the peak torque bks, that would mean me driving around at 7500rpm all the time

There two things going on here though, are you asking which is the best for mpg or which is the most efficient (i.e. best performance per unit of fuel used etc.).

I'm actually asking what is the speed at which the greatest distance will be covered per unit of fuel. I had thought that this would be directly related to engine RPM so based me question there originaly.

Edited by allgonepetetong on Monday 7th November 13:20

[5lab]

Dracoro said:

In general, as low a speed as possible but above the speed where the engine would labour.

As for the peak torque bks, that would mean me driving around at 7500rpm all the time

There two things going on here though, are you asking which is the best for mpg or which is the most efficient (i.e. best performance per unit of fuel used etc.).

this man speaketh the truth. The thing is, where an engine 'labours' is a difficult thing to define. On a petrol engine, efficiency drops off lots on engines with high/wide cam profiles, as the exhaust valve can be open whilst fuel is coming into the engine (less of an issue at higher rpm's) on a diesel or gdi engine I don't think this can happen (fuel is directly injected) - so maybe idle in 5th would be the most efficient?

There is an mpg graph for an rx8 somewhere - its best in top at around 25mph iirc

[GroundEffect]

kambites said:

It varies, but I think it's almost always between about 30 and 50mph (certainly all the cars that I've ever owned have been in that range). Typically in the highest gear that you aren't labouring the engine in.

Engines are most efficient at around peak torque, but that says little about how to get best economy out of a car - engines are not cars and efficiency is not economy.

Edited by kambites on Monday 7th November 13:17

Not around, AT peak torque.

The most fuel efficient speed is vehicle dependant.

[5lab]

GroundEffect said:

Not around, AT peak torque.

The most fuel efficient speed is vehicle dependant.

not necessarily. With things like turbos, variable cams etc, peak torque is where the most air can enter the engine, but that might not be where the car is most efficient.

[Kickstart68]

Hi

I understand the basic theory is that the engine is at its most efficient on full throttle (minimise pumping loses on a petrol engine) at the revs that equate to peak torque.

However most cars will run a richer mixture for more power on full throttle (assuming that when you use full throttle you want maximum power), so in the real world full throttle won't give best consumption, plus full throttle for most cars will land up with a far higher speed and so far higher drag.

I tend to find that what kills the consumption is accelerating back up to speed after being forced to brake. If you can just back off early, avoid braking, and then accelerate again there is no real loss, but braking is just throwing momentum away. Old carbed Alfa I had would manage about 39mpg sitting at a steady 90, but low speed stop start traffic would halve that as the accelerator pumps squirted fuel straight into the engine.

All the best

Keith

[kambites]

GroundEffect said:

Not around, AT peak torque.

Only if the engine's breathing is unrestricted, surely? So many modern engines vary boost pressure artificially throughout the rev range, that I'm pretty sure it will no longer be the case?

At least I don't see why it has to be the case.

[yellowbentines]

You really can't put an 'x'mph speed on it without knowing what car it is, mine has a 7 speed auto and sits just around 1200-1500rpm at motorway speed limits, so a slightly lower speed for me means a lower gear and higher revs therefore more fuel surely?

[allgonepetetong]

Were getting a bit OT with engine effiency.

My question is, what is the most efficient speed at which to cruise? (speed at which greatest distance travelled per unit of fuel)

[kambites]

yellowbentines said:

You really can't put an 'x'mph speed on it without knowing what car it is, mine has a 7 speed auto and sits just around 1200-1500rpm at motorway speed limits, so a slightly lower speed for me means a lower gear and higher revs therefore more fuel surely?

Only if the greater frictional losses due to the higher number of rotations of the engine per unit distance equate to more wasted energy than is saved through lower drag. There is no reason at all to assume that your car will be most efficient in top gear. No car I've ever owned has been.

[300bhp/ton]

allgonepetetong said:

I was driving home on Sunday morning after stayin with friends, and the petrol guage was a little lwer than I had anticipated. I had 65 miles to drive on the M40 and M25 and the trip computer was showing I had 80 miles range.

This should be fine but I know the trip computer undereads massively for MPG so I thought I would set the cruise at 65 to ensure i could get home without running out of fuel.

This got me thinking, what would be the most fuel economical speed / engine RPM to travel at?

I'm thinking that peak torque would be most efficient, but this it at 4500rpm in my car and in top that's around 90mph. I could cruise in 5th at around 75mph but that's defeating the point as I would be travelling for 8 more minutes and burning fuel all that time.

So, is top gear at peak torque really my most eficient (90mph) cruising speed?

52.845623mph given an 11.225547mph tail wind on a level gradient at an average elevation of 474.55548745 feet.

[kambites]

allgonepetetong said:

Were getting a bit OT with engine effiency.

My question is, what is the most efficient speed at which to cruise? (speed at which greatest distance travelled per unit of fuel)

And the answer is "it depend on the car but probably somewhere around 40mph". At least in my experience.

[5lab]

petrol engines suffer from throttle losses as well, but I can't figure out if they're worse at lower rpm or not??

[Kickstart68]

allgonepetetong said:

Were getting a bit OT with engine effiency.

My question is, what is the most efficient speed at which to cruise? (speed at which greatest distance travelled per unit of fuel)

Will depend on the car, which will depend on gearing, wind resistance, how the engine is tuned, etc.

Used to be that a lot of boring cars were optimised for 56mph / 90kmh so they looked good on the old official mpg figures.

All the best

Keith

[kambites]

Kickstart68 said:

Used to be that a lot of boring cars were optimised for 56mph / 90kmh so they looked good on the old official mpg figures.

I've heard people say that before, but as far as I know it's never actually been true? I think it would be very hard to tune a car to be most efficient at a speed that high. It may be the case that manufacturers stopped caring about economy above 56mph, but I'd be amazed if cars were most economical that that speed.