If the engine is hot rodded a bit both idle speed and redline will be higher than stock. A smaller crank pulley can get accessory speeds back to normal. I run the 6.25" March pulley in place of the 7.25" stock on my "warmed up" BB and figure accessory speeds are pretty close to stock with the higher RPM engine. If you go too far with the smaller pulley and really slow down the accessories, you could get into cooling and charging problems. In my case the 6.25" pulley works great for charging and cooling but each application is different.