Mustang underdrive pulley
systems increase you Mustang engine horsepower by reducing the amount of power required to drive external engine accessories. For example, the alternator, power steering, water pump and air compressor rob horsepower from the engine. By replacing factory-sized crank and accessory drive pulleys with a resized pulley, the accessories are slowed, resulting in more horsepower available to the drive wheels. Typically, a set of Mustang underdrive pulleys will increase you Mustang's horsepower
by 12-15, and increase mileage by 2 to 3 MPG.