Superchargers force more air into an engine's combustion chamber, which increases power output. This allows smaller, lighter engines to produce the same power as larger engines, and a lighter vehicle gets better gas mileage. At moderate RPMs, a supercharger will force more air into the engine than a normally aspirated engine, which further increases fuel efficiency.

On the Other: Drag At Low RPMs and Shorter Engine Life

At low RPMs, a supercharger that gets power from the crankshaft will create drag, thereby lowering gas mileage. Additionally, any power-enhancing modifications to your engine will increase engine wear to a degree, which can shorten the overall life of your engine.

Bottom Line

A supercharger increases gas mileage at moderate RPMs, but may come at the cost of increased engine wear. However, engine wear and fuel mileage are primarily affected by an individual's driving habits and engine tuning. By avoiding conditions that require high-performance from your engine and keeping up on engine maintenance, you can increase the efficiency of your car with a supercharger without sacrificing engine life.

Source:

SuperchargersOnline.com

Magnuson Products

More Information:

Muscle Mustang & Fast Fords.com: All About Superchargers