You have to burn a certain amount of fuel to make a certain amount of hp. You need to make a certain hp to maintain a certain speed. A bigger engine just has the capability to burn more fuel to make more hp to go faster. In the real world, it doesn't work out this simply, varying efficiencies of varying engine designs to come into play. This El Camino was geared to run 1500 rpm at 70 mph and the engine was cammed to make huge low end torque. The motor only made 300 hp at the wheels, but had well over 600 ft lbs of torque from 1500 rpm up to 4200, where the owner set the redline.