A supercharger sends compressed air into the combustion chamber to maximize the fuel burn and enable a greater consumption of fuel for a given size of engine. While burning more fuel can hardly be considered good for fuel economy, the supercharger does enable larger vehicles to be powered by small motors, and the weight savings is significant. There are additional positive effects of a supercharger on fuel economy, and supercharged cars can be very enjoyable to drive, making them a fine alternative to consider when comparing vehicles.
Compressed Air Makes for an Efficient Burn
It might seem counterintuitive, but a car's engine has a tough time burning all the fuel that enters the cylinders. Modern advances such as direct injection help, but there is always some portion of unused fuel that is sent out the tailpipe with the exhaust. How much of the fuel gets burned depends on the fuel/air mixture, and the compressed air that is delivered by a supercharger optimizes that mix. This really accounts for the first level of fuel savings because all unburned fuel is wasted.
Bigger is not Better
An additional effect of having compressed air in the combustion chamber is that you can burn more fuel in each piston cycle. Burning that extra fuel amounts directly to having more power available without making the engine bigger. Small supercharged engines are able to produce horsepower ratings similar to those of engines twice their size or more, depending on how they are configured. In practice, this means that a larger car like a turbo Volvo wagon (turbochargers are a form of supercharger) can have a smaller motor and higher fuel economy ratings due to the reduced weight. At the other end of the spectrum, a smaller, lightweight car such as the Mini Cooper S can deliver high performance and good fuel economy at the same time.
Low RPM can also equal fuel savings
Supercharged engines are able to make good power at the low end of the RPM (revolutions per minute) range. For any given speed your car is traveling, your will burn less fuel at low RPM as opposed to high RPM, as the engine needs more fuel to spin faster. Cars will often have transmissions with five and six speeds or more to maximize this effect. It's not uncommon for vehicles so equipped to cruise at highway speeds with the engine RPM barely over engine idle speed.
Good Fuel Econmy is not Always Straight Science
If there is a downside to having a supercharger, it comes in the form of temptation. Due to the nature of a supercharger's engineering, it makes more power as the RPM raises. That linear increase in the power combined with vehicle momentum is a thrill that can be very hard to resist. Your first few weeks driving a supercharged car will not be your most economic. With time and sound judgment, you can train yourself to resist the lead foot and your fuel savings will improve.
A Hidden Benefit is Towing
A lot of families own a second vehicle like a pickup or a van to handle utilitarian tasks like hauling mulch or oversized toys. Your supercharged car may well have enough power to tow a small utility trailer and prevent the need for the fuel-thirsty truck. Besides being able to run to the home center, you could also pull a small camper like a pop-up or a boat. Not only will you be able to enjoy getting the use of two vehicles for the price of one, but you'll be cruising in style with the amenities and safety features of your car while you are at it.
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