Why couldn’t your car engine run at, say, 2,700 rpm all the time, about the same as that of many piston aircraft engines producing full power? Reciprocating aircraft engines generally spin a propeller via direct drive, with the prop essentially bolted to the crankshaft. Typically, piston aircraft engines produce peak power output at a redline near 2,700 rpm. But compared with car engines, aircraft engines produce gobs of torque at relatively low rpm. Auto engines, on the other hand, usually produce very little torque at those settings, while also relying on liquid cooling. They’re generally designed to produce their peak power at settings above 6,000 rpm, and so to work they need a reduction gear box.