A good site to check out is:
http://www.tciauto.com/tech_info/speedo_gears.htm
By the way, your tire diameter should be 32.83", so the only other factors in the equation are the number of teeth on the drive gear and the driven gear. It's simple to pull the driven gear housing and count the teeth, but you will most likely need to pull the tail housing to count the drive gear teeth. In any event, it is quite easy to pull the tail housing to count teeth and/or replace the drive gear. Good luck!