[debergalis]

Aircraft piston engine efficiency is actually quite similar (measured in brake specific fuel consumption) to modern auto engines. The Lycoming O-320 in a Cessna 172 burns 280 g/kWh. Modern auto engines like the EcoBoost burn not much less -- ~250 g/kWh. See https://en.wikipedia.org/wiki/Brake_specific_fuel_consumptio....

Moreover, aircraft engines like the O-320 can run for hours at continuous max power output. Good luck doing that with a car engine.

Car engines absolutely demolish aircraft engines on emissions, though. They have O2 sensors, computer-controlled ignition systems, and catalytic converters, whereas aircraft piston engines have fixed ignition timing and burn extremely rich of peak (meaning massive emissions) at takeoff power. And on top of that, while little engines like the O-320 can burn ethanol-free autogas, the big piston engines like a Continental IO-550 require leaded fuel, adding another pollutant to the mix.