Originally Posted by DEATH2000
Underdrive pullies are designed to have a lighter mass over stock allowing the belt to spin free'er (sp?).
Not exactly . . well, yes they're lighter, and that's a plus. But more importantly, they're slightly larger in diameter than stock pulleys, making it easier for the crankshaft to turn them. The result is a reduction of parasitic drag, and a slower rotating speed for your accessories without any adverse effect. Simple physics