Propulsive efficiency is a different matter. It has two components: the engine and the propeller. (In the present state of the art, only propellers, not jets, can achieve high efficiency at low speed.) Propeller efficiency, at best, is around 85 to 90 percent. The efficiencies of internal-combustion engines range from 25 to 35 percent. What this means is that, of the energy in the fuel you use, only about one-fifth propels your airplane; the rest merely warms the atmosphere in your wake. Some of the heat lost is in the exhaust; some extracts a double penalty, because it adds cooling drag as well.