Originally Posted by
fstshrk
I find going to 25mph from 20 mph to be quite harder than going from 15 to 20 mph. The relationship I think varies with the square of the speed.
As per the post above yours, it's asymptotic to the cube of the speed because aerodynamic drag increases as the square of the speed. The drag exerts a ******ant force, the power required to overcome that force therefore rises as the cube (because power is force x velocity).
Above about 40 kmh aerodynamic drag dominates all other forces.