The driven pully rpm equals the crankshaft pully rpm multiplied by the crank pully diameter and divided by the driven pully diameter. Obversly, the crank rpm equals the desired driven rpm multiplied by the driven pulley diameter and divided by the crank pulley diameter.
For alternators make sure that small frame alternators are not driven higher than 10,000 rpm at the highest engine
rpm. For large frame alternators limit their spindle speed to 7,000 rpm. For longevity of belts do not use alternator
pulley diameters less than 2.4-2.5 inches.