Each of our high output alternators come with smaller overdrive pulley. This pulley is made to increase low RPM charging for maximum output. It is custom for the diameter of the overdrive to be less than your factor pulley, depending on the kind of vehicle you own. For this reason, unless otherwise specified, you will need at least a 1/2 inch shorter belt. You can purchase this from any auto parts store. In using the belt and the pulley system, ensure that each part of the pulley system is free of oil or grease, this will help its efficiency.