Why is 56mph more fuel efficient than 40mph?
Discussion
It’s always recommended by the experts that the most efficient speed to drive at is 56mph.
Surely driving at 40mph or 20mph for example would be more efficient? (Air resistance increasing with square of velocity etc etc).
(Provided the you are in the right gear, engine isn’t labouring etc. )
Surely driving at 40mph or 20mph for example would be more efficient? (Air resistance increasing with square of velocity etc etc).
(Provided the you are in the right gear, engine isn’t labouring etc. )
It depends on a lot of factors. Gearing, power band, aerodynamics, etc.
Excuse the random made-up figures, but as an example, let's say a car requires 40% more fuel per minute to cruise at 60mph than it does to travel at 40mph.
Yes, it will use 40% more fuel over the course of a minute, but it will also cover 50% more distance in the same time frame, meaning MPG would be better at the higher speed.
These figures probably aren't accurate at all but just an example!
Excuse the random made-up figures, but as an example, let's say a car requires 40% more fuel per minute to cruise at 60mph than it does to travel at 40mph.
Yes, it will use 40% more fuel over the course of a minute, but it will also cover 50% more distance in the same time frame, meaning MPG would be better at the higher speed.
These figures probably aren't accurate at all but just an example!
Edited by MrGTI6 on Sunday 13th March 15:55
It obviously can't be true that every car in every situation is at its most efficient at 56mph. For a start, some cars can't even drive at 56mph.
I can't remember where I saw this, but isn't it essentially the lowest speed you can drive smoothly at in your highest gear? There's a reason why modern cars nag you incessantly to shift up. But that still only gives you the best instant efficiency, you have to think about lots of other factors like how much you brake (braking is wasted fuel) to work out your most efficient approach to driving overall.
I can't remember where I saw this, but isn't it essentially the lowest speed you can drive smoothly at in your highest gear? There's a reason why modern cars nag you incessantly to shift up. But that still only gives you the best instant efficiency, you have to think about lots of other factors like how much you brake (braking is wasted fuel) to work out your most efficient approach to driving overall.
I find its typically the lowest speed in the highest gear, while not lugging the engine
I find 30mph, in 4th, 40 mph in 5th and 50 in 6th are the points which my cars (hatchbacks) are most efficient
50 is more efficient than 56. but it depends on your gearing and aerodynamic drag. A hatchback I suspect will sharper drop in MPG than a saloon / coupe as the speed increases.
for reference the MPG vs speed follows a strong square law relationship. I tested this on my own car.
The lower gear you are in the greater % of losses are internal frictional, I suspect this is why the highest gear is often the best
I find 30mph, in 4th, 40 mph in 5th and 50 in 6th are the points which my cars (hatchbacks) are most efficient
50 is more efficient than 56. but it depends on your gearing and aerodynamic drag. A hatchback I suspect will sharper drop in MPG than a saloon / coupe as the speed increases.
for reference the MPG vs speed follows a strong square law relationship. I tested this on my own car.
The lower gear you are in the greater % of losses are internal frictional, I suspect this is why the highest gear is often the best
northandy said:
I thought the 56mph was from the old way they used to measure mpg, it’s not the most efficient just a set speed that manufacturers had to measure at.
Correct.The old test standard was:
GENERAL SPECIFICATIONS
3.1. Fuel consumption shall be determined by the following tests:—
3.1.1. Cycle simulating urban driving, as described in Annex 4 to this Regulation
3.1.2. Constant speed test at 90 km/h
3.1.3. Constant speed test at 120 km/h
Largechris said:
It’s always recommended by the experts that the most efficient speed to drive at is 56mph.
Surely driving at 40mph or 20mph for example would be more efficient? (Air resistance increasing with square of velocity etc etc).
(Provided the you are in the right gear, engine isn’t labouring etc. )
56 is just the most economic of the old official mpg figures that the experts had to choose from. 56mph was used because it corresponds to 90kmh which is a common speed limit in Europe. Surely driving at 40mph or 20mph for example would be more efficient? (Air resistance increasing with square of velocity etc etc).
(Provided the you are in the right gear, engine isn’t labouring etc. )
mintmansam said:
I find its typically the lowest speed in the highest gear, while not lugging the engine
I find 30mph, in 4th, 40 mph in 5th and 50 in 6th are the points which my cars (hatchbacks) are most efficient
50 is more efficient than 56. but it depends on your gearing and aerodynamic drag. A hatchback I suspect will sharper drop in MPG than a saloon / coupe as the speed increases.
for reference the MPG vs speed follows a strong square law relationship. I tested this on my own car.
The lower gear you are in the greater % of losses are internal frictional, I suspect this is why the highest gear is often the best
From 55 to 80mph your MPG halves? I find 30mph, in 4th, 40 mph in 5th and 50 in 6th are the points which my cars (hatchbacks) are most efficient
50 is more efficient than 56. but it depends on your gearing and aerodynamic drag. A hatchback I suspect will sharper drop in MPG than a saloon / coupe as the speed increases.
for reference the MPG vs speed follows a strong square law relationship. I tested this on my own car.
The lower gear you are in the greater % of losses are internal frictional, I suspect this is why the highest gear is often the best
I can't think of any cars I've owned having such a drop off.
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