We do a lot of driveline swapping be it engines, trans, or rear ends and have found the easiest for us is to use a GPS to get an accurate speed and then compare that to the speedometer reading. Say the Gps shows 55mph and the speedometer reads 65mph, that's about a 15.5% error so the DRIVEN gear neads slowed dowm 15.5%. On older or rare trans the easiest is to let some place like the Speedometer Shop make you up a correction unit. On popular trans a wide selection of both drive and driven gears can be found at dealers or places like TCI and The Transmission Shop. Google "speedometer gears" and there is a large offering. You can pick a set that gets you where you need to be. Most of our stuff is street or resto rod so we have been using the GPS speedometers now available. Transmission type, tire size, or rear gearing has no effect on a dead accurate reading so the guys that street drive during the week and race on weekends can switch out anything they want and not have to worry about how fast the're actually going when meeting John Law.
