There's quite a lot of energy needed to just turn the engine over in an ICE car so if you are driving a 5L car at 20mph most of the fuel use is just turning the engine rather than driving the car. Faster, more goes to driving the car.

ICE engines have an efficiency map of output torque/RPM and (up to a point) higher torque usually means more efficient use of fuel. In most ICE vehicles the engine efficiency gains balance with the drag efficiency losses between 40 and 50 mph.