The power required to push your car through the air is proportional to drag. Drag depends on the "drag coefficient" CD, densitiy of the fluid, velocity squared and frontal area S. If you double your speed, you require four times the energy to go through the air. Sure you'll get their in half the time, but the quadratic term dominates. Maybe your engine's efficiency will be 10% or 15% better at one speed or another, but that will be totally dominated by the velocity squared term.
see above posting Nr. 17 for a practical demonstration on what you need for V equals 24.4 m/s.