1/6/2014 · 6. Diesel engines get best performance below 65 miles per hour. They get peak power when the engine revolutions per minute ( RPM) are low, generally at speeds below 65 miles per hour. Gasoline engines, in contrast, get to peak power by running the engine fast and high and the rpm at 5, 000 (i.e., with the pedal to the metal). 7.