Performance pulleys (also called underdrive pulleys) work by driving a vehicle's accessories at a slower rate than previously set factory settings. The controlled speed means the engine spends less effort spinning the performance pulleys and the other gadgets attached to it. The reduced workload on the engine translates to extra power given to driving the wheels forward.
On the average, a performance pulley can free up as much as 15 hp (horsepower)while costing just a few hundred bucks or less. Most after-market auto parts dealers carry a wide variety of underdrive pulley configurations which allow the car owner the option to tweak or alter the amount of underdrive built into the pulley set. This is a very handy feature for those who want to run a full-speed alternator while still getting gains through the power steering and air conditioning pulleys.