Petrol economy and wear and tear.

[Potatoman]

Is 56mph really the most economical speed for a car like what i keep hearing? I have a 1.4 litre Peugeot 206 LX. What is the rough difference in petrol consumption from 56 mph anf 70 mph? Is wear-and-tear significantly more at 70mph?

[KebabMeister]

It depends on whether the car manufacturer has designed their car to be most efficient at 56 mph.  It comes down to gear ratio and how the engine performs whilst in top gear at around 56 mph.  This is generally the speed that is accepted as the 'standard' commuting speed. 

Obviously other things are to be taken into consideration such as car load (passengers), roof racks, wind, gradients etc.

[Tim Baxter]

The most economical speed to run a car at is the speed at which the engine will support the highest gear.  This is because the slower the engine is turning the less fuel it uses.  It is true that at certain revs engines produce the most efficient power per rev but I believe that it does not gain enough in efficiency to warrant a higher speed to become most fuel efficient, I could be wrong......