Aircraft engines typically operate at slightly better efficiency than car engines, mainly because they are operating at a higher percentage of full power than a car engine, as others have me mentioned. All engines are more efficient when this is the case. in addition, low rpm engines with fewer cylinders are generally more fuel efficient due to lower internal friction.
Why don't aircraft get better miles per gallon, despite being much lighter than a car? Two reasons: one is that aircraft go fast, and the energy required to overcome parasite drag over a certain distance rises as the square of speed. The second is that induced drag (the drag that comes with holding the aircraft in the air) is high in relation to rolling friction, which is the equivalent energy loss for a car (or train).
Flying at high altitude helps - the engine efficiency decreases but that is overcome by much lower aerodynamic drag.