There's airplanes that are comparably fuel-efficient (per mile) to cars, such as a number of motorgliders and light-sport aircraft. Additionally, they often can use the cheaper MoGas instead of AvGas.
I've just bought the plans for Michel Colomban's luciole - 81 knots on 4.5l/hr or about 90 miles per gallon. The thing is, it's not really fair to compare this small single seater with no luggage capacity to e.g. a toyota prius. In automotive terms, it's equivalent would be a concept-car for commuters that might do 200-300 mpg.