If you looked at Tech's post #6 above and opened the link you've seen that on the TH350 there are eight different drive gears, which are internal to the tranny tailhousing, and then seventeen different driven gears, which come out easily. You need to know which drive gear you have, too, unless Mike has some way of calculating that it doesn't come into play. On that link if you simply plug in the drive gear tooth count, the axle ratio and the tire diameter calculated from the circumference it will tell you which driven gear you need with no hassles and eliminating any guesswork.