There have been many good discussions here regarding this and you'll benefit from doing a "search"...but from what I understand the main pully to replace with an underdrive in order to reap HP is the crank pully, and that's the only on on our Bullitt's that's stock GT size! So some guys are ordering the Steeda set and just doing the crank pully, while others try to just buy an underdrive crank pully (usually they come in sets, though)