Surely interia must play some part of this conumdrum?
To get the car to 70 rather than 60 must use more fuel but only an imeassurable amount.
Kinetic energy is ½mv², and let's assume your car is 1500kg. 60mph is ~27m/s; 70mph is ~31m/s.
At 60mph, your car has a kinetic energy of ½×1500×27² = 547kJ. At 70mph, it's ½×1500×31² = 721kJ, a difference of 174kJ.
Internal combustion engines run at an average efficiency of about 20% and a maximum of no more than 40%. As high load is where engines are more efficient, let's use 33.3% (1/3) as the number to make the maths easier. Ignoring friction and additional wind resistance for the moment, our engine needs to burn 174×3 = 522kJ of fuel to generate the required additional kinetic energy.
Petrol has an energy density of 32MJ/litre; diesel is around 36MJ/litre; 522kJ is thus somewhere between 1/60 and 1/70 of a litre; about a tablespoon extra over the time you take to change speed.
So accelerating from 60mph to 70mph costs you about 2p on top of the fuel you're using to maintain the 60mph in the first place.
522kJ is about 26g of Mars Bar; a so-called "fun size" Mars Bar is just under 20g.
When the car is at 70 would it not then take the same amount of mpg to keep it there?
With frictionless bearings and tyres in a vacuum, yes, it would. As already stated, the majority of drag on a car at speed is wind resistance.
If you've ever ridden a bicycle with multiple gears, you'll realize that on the flat, your maximum speed is not regulated by your legs' maximum rotation. A bike and rider are pretty poor aerodynamically, so at only 20mph, 80% of the energy exerted by the rider is in overcoming wind resistance, and that goes up with the square of the speed.
A car is in exactly the same position, but with better aerodynamics and stronger "legs", the numbers are just a bit higher.
If your postulate were correct, cars would be unlimited in their top speed — a small engine would eventually provide the energy to get to any speed and hold it there. That's clearly not the case, though, which is why you need the best part of a thousand horsepower to get to 250mph.
The optimum speed for economy in any car is slow: I'd guess around 30mph depending on the gearing. The myth that 55 or 56mph is optimum comes from the old days of fuel consumption measurement when figures were quoted at constant 56 and constant 75mph. Many cars' designs (engine/gearing matches) were influenced by the desire to give good 56mph figures, but that's still too fast for best economy as the wind resistance is dominating at that speed.
In summary, drive at a speed you're comfortable with, bearing in mind that the faster you go, the worse your economy gets at a worse-than-linear rate.
Veryon in front of me.
