Forced induction engines, such as those with turbochargers and superchargers added, are typically less fuel efficient than the same sized air aspirated engine, regardless of the vehicle make. However, turbochargers can make some smaller engines more powerful and more fuel efficient than their larger counterparts.
Both devices use the same basic principle of forcing compressed air into a vehicle's engine cylinders to increase compression ratio. Typically, the amount of oxygen introduced into the combustion chamber increases the fuel usage proportionally. The efficiency difference between the two is caused by their designed method of generating the power to operate. Superchargers are belt driven compressors that use engine power to create more engine power. Turbochargers use wasted energy, in the form of vehicle exhaust, to generate additional horsepower, which makes them more fuel efficient.
A turbocharger installed on a smaller engine, such as a four-cylinder or six-cylinder motor, can give it more horsepower while using less fuel than a six or eight-cylinder engine, respectively. Turbochargers generally cause no fuel usage difference than the same sized conventional engine, when the vehicle is operated at low speeds or idling.
Superchargers are typically used only on larger engines, including six-cylinder or eight-cylinder models, to generate increased power for heavy load applications, such as towing. Because they use so much mechanical energy from the engine to produce the additional horsepower any fuel efficiency benefits are usually negligible.