Best mpg = Slowest speed tallest gear
The fact is you will need "x" power to go "x" speed
Since Drag as stated before increases by the square of speed the engine needs to produce much more power to go those extra 10mph.
I built an electric motorbike and that was calculated power using Watts used to go at a specific speed. (768watts = 1hp appx).
Now in order to do 15mph I only needed 300-400watts. To do 30mph I needed 1200-1500w so to double my speed it cost me four times as much in electric. Every time I increased speed I had to take larger and larger batteries.
Its the same with Engines. You can not get around the physics. You can lower the drag which is the only way to really increase mpg or simply just go slower.
Engines them selves are only around 80% efficient. Then you get transmission losses.
Electric cars are the same but can be up to around 90% but you still loose energy in thermal losses.
This is why the most fuel effcient car's tend be be those that create less drag or have been designed that way. Hence the big push on sealing up vents and radiators. Aerodynamic mirrors, lighter weight.
Last edited by scottyf; 11-09-12 at 11:53.