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.

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.