Backpacker has it.
Aircraft engines - at least our piston engines are only marginally less economical than car engines. However they run at significantly higher power factors and so the fuel gets used to create the amount of power required.
An interesting comparison is a Toyota Prius (or otherwise the Pious). Not expecially economical as a car - put it on a teack and use the maximum power it can generate and you'll get lss than 10 mpg - and pretty poor handling.
The capcity of our piston engines allows the power to be generated at lower rpm - so allowing direct coupling to the propeller and no gearbox. It also gives large combustion chambers and generally aids combustion - hence the good values of specific fuel consumption that aero picton engines have.
The obvious economy of car engines is down to managing the power changes very cleverly and ensure no fuel is burnt on the overrun, when stationary, when warming up etc etc.