Fuel economy at 65 mph vs 60 mph

The reason objects have a terminal velocity when falling through the air is because they reach a speed where drag increases to a level where they can no longer accelerate. That's why parachutes work. Going faster requires more power and more fuel.

The 4100 Lb Bugatti Veyron requires 1000 hp to reach its top speed of over 250 mph and burns 1.4 gal per minute in order to stay there.