well, you may be able to change just the driven gear, but it would be really helpful to know the tooth count on the drive gear too..but for example..lets say your driven gear was a 36 tooth---now you are going to be spinning the driveshaft faster, so your speedo is gonna read faster---now you will need to add teeth to your driven gear to slow the speedo needle back down to where it should be..say going to a 40 or 41 tooth. note that on a th400 there are 2 different driven gear adaptors..i think one for 34 to 39 tooth and one for 40 to 45 tooth. easy to find on the net. tci has a good gear calculator on their page, but from some experiences, if you calculate you need a 40 tooth driven gear, many have found their speedos to be more accurate going up one tooth. if you have a gps, this really rocks in helping see your true mph cruising and calibrating your speedo..no reason with some testing it cant be practically dead on...
