With the addition of a supercharger kit, your average miles per gallon (MPG) will change. That change can be an increase or decrease in fuel economy depending on your driving habits and application.
Often an increase in fuel economy is seen under standard driving conditions with the addition of a supercharger. A 2 to 3mpg increase is not unusual under normal daily driving conditions. This gain can be attributed to the supercharger reducing the pumping loss of the engine. This loss can be better described as the vacuum force produced by the engine that is required to actually pull the air/fuel mixture into the cylinder. The supercharger helps this process by equalizing cylinder pressure and effectively helping to ‘push’ the piston to the bottom of the intake stroke, increasing engine efficiency.
One reason owners often say there is a loss in fuel economy when adding a supercharger kit to their car is that most owners find it difficult to keep their foot off the mat! Under supercharged racing conditions, fuel economy decreases due to the engine’s increased ability to produce horsepower and torque. This increase is generated by consuming a greater quantity of fuel and air. Simply stated if you drive hard or race often your average miles per gallon will decrease. This is similar to the fuel economy loss seen in a naturally aspirated engine under extreme driving conditions.