actually its an under dirve crank pulley..
thats the name of it and the info is as follows:
"An underdrive pulley system for your Ford Truck will increase engine horsepower and IMPROVE GAS MILEAGE by reducing the amount of power required to drive external engine accessories on your Ford Truck. The alternator, power steering, water pump and air conditioner all rob horsepower from the engine. When you replace the factory-sized crank and accessory drive pulley on your Ford Truck with a resized underdrive pulley, the accessories are slowed, resulting in more horsepower available to drive the rear wheels. Typically, an underdrive pulley will increase output for your Ford Truck by 12-15 horsepower, resulting in quarter mile improvements of .2 to .25 seconds. In addition, mileage improvements of 2 to 3 mpg are typical."
but i want actual real input from users regarding this.
E30 BMW 325is - 2.7L Stroker - Custom 4 piston brembo setup - XXR 522s - 272/248 Cam (for quick spool) - Ireland Engineering Rocker Excentrics - Sport Shocks/Springs - Ireland Engineering Sways - Ported Manifolds - SOON Megasquirt3 - HX35 - Custom Manifold Adapter - 440cc