For propeller aircraft, drag is minimised when the lift-to-drag ratio is maximised. However, the speed for this is typically regarded as too slow, so propeller aircraft typically cruise at a significantly faster speed.[6] Combustion engines have an optimum efficiency level for fuel consumption and power output.[7][better source needed] Generally, gasoline piston engines are most efficient between idle speed and 30% short of full throttle. Diesels are most efficient at around 90% of full throttle.[8][better source needed]