This The 5 inch pulley should do it. You are absolutely correct in assuming that one of about the same size as is used in the car about a 6 would be correct to give the right alternator speed. But remember that most cars spend much of their lives well BELOW 2500 rpm, so the pulley is sized to give a compromise belt speed for water pump, fan, alternator, etc. In fact many cars have a stepped pulley with a different size for running the Power Steering pump, and yet another for the A/C. -384 Extremely informative great vid. "home energy monitor uk " and "home energy station price " (include "perm motor pmg 132 permanent magnet dc", "magnet battery motor ", "fixed magnet motor ", "how to make a watermill ", "15 kw wind generator ", "Home energy audit ohio ", Solar Training Courses are creating job ready skills and a stable career path for thousands of people in the Renewable Energy industry. & How is electricity produced from wind )