I'm bulking a motorized bike with a 3hp, 79 cc Harbor Freight engine that's on sale for 79.99. Max rpm is 3600 without making internal mods. I'm using a 12 tooth clutch off the engine to a jackshaft in. The jackshaft out is the standard bike gears (the large three sprockets in the front) running to the usual rear freewheel gears. I don't know the exact tooth counts but the bike is a standard Huffy Blades. Gears are average sized. My question is -- what size should my jackshaft in sprocket be? I want to be able to go ~40 mph or so, but still have enough power to climb hills. What size sprocket should I use (tooth count) and how fast can I expect to go on it. I'm a 130 lb rider, and the hills we have are quite steep -- typical New England countryside if you know what I mean.