At which speed is a car at its most fuel efficient?
Discussion
Apologies if this has been discussed before.
Is there a general rule or formula at which a car driving at a particular speed (I presume in the highest gear?) will be at its most fuel efficient?
I understand that this speed is likely to be different for a 1.2L 4pot Hyundai versus a 4.2L V8 Jaguar, but what do the drivers have to be doing so that whilst making progress, their respective engines are drinking the least fuel?
In a 5 speed manual car, could you possibly get more mpg driving in 4th gear than 5th??? i.e. at a speed suitable for 4th gear in that vehicle. Or must you be in the highest gear?
Where does torque come into the fuel efficiency equation?
Obliged much.
Is there a general rule or formula at which a car driving at a particular speed (I presume in the highest gear?) will be at its most fuel efficient?
I understand that this speed is likely to be different for a 1.2L 4pot Hyundai versus a 4.2L V8 Jaguar, but what do the drivers have to be doing so that whilst making progress, their respective engines are drinking the least fuel?
In a 5 speed manual car, could you possibly get more mpg driving in 4th gear than 5th??? i.e. at a speed suitable for 4th gear in that vehicle. Or must you be in the highest gear?
Where does torque come into the fuel efficiency equation?
Obliged much.
In the swift 5th gear at about 50mph got many miles to the gallon, averaged 60 once doing this on 80 mile trip! also slip streaming lorries helps 
In the TT 6th gear at about 60 was nice, about 2000rpm in the audi and 1800 in the swift was good for the economy and not labouring the engine.
In the TT 6th gear at about 60 was nice, about 2000rpm in the audi and 1800 in the swift was good for the economy and not labouring the engine.There are two key factors; how late you are and how far away your meeting is;
Do I put the hammer down and risk a fuel stop which will cost time?
Or cruise gently which will feel slower but may get with there without stopping
Actually, I think it's all to do with:
speed
peak torque
gearing
how easily the car cleaves through the air / rolling resistance
Do I put the hammer down and risk a fuel stop which will cost time?
Or cruise gently which will feel slower but may get with there without stopping
Actually, I think it's all to do with:
speed
peak torque
gearing
how easily the car cleaves through the air / rolling resistance
Based on the onboard mpg dialthingy, in my 1.2 clio its about 37mph in 5th or 24mph in 4th. Both are pretty identical. I'd suggest it is at a point where the revs are at their lowest, without labouring the engine. Generally in 5th gear (not 6th as air resistance will be increacing and if there was any sort of incline the engine will struggle (at least that is what I found with the insignias and meganes I have driven).
Audi 525i said:
Apologies if this has been discussed before.
Is there a general rule or formula at which a car driving at a particular speed (I presume in the highest gear?) will be at its most fuel efficient?
I understand that this speed is likely to be different for a 1.2L 4pot Hyundai versus a 4.2L V8 Jaguar, but what do the drivers have to be doing so that whilst making progress, their respective engines are drinking the least fuel?
In a 5 speed manual car, could you possibly get more mpg driving in 4th gear than 5th??? i.e. at a speed suitable for 4th gear in that vehicle. Or must you be in the highest gear?
Where does torque come into the fuel efficiency equation?
Obliged much.
there are too many factors.Is there a general rule or formula at which a car driving at a particular speed (I presume in the highest gear?) will be at its most fuel efficient?
I understand that this speed is likely to be different for a 1.2L 4pot Hyundai versus a 4.2L V8 Jaguar, but what do the drivers have to be doing so that whilst making progress, their respective engines are drinking the least fuel?
In a 5 speed manual car, could you possibly get more mpg driving in 4th gear than 5th??? i.e. at a speed suitable for 4th gear in that vehicle. Or must you be in the highest gear?
Where does torque come into the fuel efficiency equation?
Obliged much.
you would need to know the air resistance/ weight/ power/ tyre resistance/ power developed at x revs/ and prob a few more
Efbe said:
there are too many factors.
Agree completely.Efbe said:
you would need to know the air resistance/ weight/ power/ tyre resistance/ power developed at x revs/ and prob a few more
I am really wishing to know at what revs and what gear I should be driving to achieve maximum fuel efficiency (assuming I keep the tyre pressures correct, weight down in the gear, and have the car correctly maintained).Audi 525i said:
I am really wishing to know at what revs and what gear I should be driving to achieve maximum fuel efficiency (assuming I keep the tyre pressures correct, weight down in the gear, and have the car correctly maintained).
the other very big factor would be the Cd (air resistance) of the car.I drive a Hyundai i10 which has a 30 litre fuel tank (some of you drink more than that on a good weekend).
I once got 330 miles out of a single tank but that was essentially 2 motorway journeys at a constant 60 mph (bang on 50mpg).
As an experiment, I did the same 2 motorway journeys, this time at a constant 70 mph but got only 275 miles out of a full tank (around 42mpg) - a 16% drop in efficiency.
It would be unwise and probably unsafe to do this journey at 50mph to see what effect that has on the mpg figure.
I once got 330 miles out of a single tank but that was essentially 2 motorway journeys at a constant 60 mph (bang on 50mpg).
As an experiment, I did the same 2 motorway journeys, this time at a constant 70 mph but got only 275 miles out of a full tank (around 42mpg) - a 16% drop in efficiency.
It would be unwise and probably unsafe to do this journey at 50mph to see what effect that has on the mpg figure.
hornetrider said:
Anyone got any string?
I have, but I'm afraid I'm not sure it's long enough for your needs. Can anyone help? Peak torque isn't necessarily equal to peak thermal efficiency unfortunately, you really need a lot more data to work this out for an individual car to work this out than is generally available to the average person - BSFC (brake specific fuel consumption) plot, and the aerodynamic drag coefficient as a bare minimum (BSFC plot tells you how efficiently the engine is working - i.e. what percentage of the fuel's energy is turned into crankshaft energy - as you vary revs and throttle position). To give you an idea, http://www.inference.phy.cam.ac.uk/withouthotair/c... shows fuel consumption for a 318 and a prius - the beemer has the highest mpg somewhere between 60 and 110km/h, the prius at much lower speeds.
It'll be at some point where Air resistance (which cubes with speed) is sufficently low, and the engine is tunring low but not laboured revs in a high gear.
You need some speed to get the miles part of the equation, and everything else needs to pitch in the favor of reducing the gallons used - Low drag, lowengine speed, high engine efficiency, long gearing, low rolling resistance/transmission losses (which also increase with speed)....
It's 52.4Mph in 5th gear.
You need some speed to get the miles part of the equation, and everything else needs to pitch in the favor of reducing the gallons used - Low drag, lowengine speed, high engine efficiency, long gearing, low rolling resistance/transmission losses (which also increase with speed)....
It's 52.4Mph in 5th gear.
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