Use the following formula to find the number of teeth needed on your speedometer driven gear.
B = Miles traveled according to odometer
A = Actual miles traveled (per mile markers)
T = Number of teeth on driven gear used to calculate miles traveled
(B/A) * T = Number of teeth needed on driven gear (round off to nearest whole number)
Example: B = 100, A = 114, T = 29 (100/114) * 29 = 25.4 rounded to 25
When rounding decimals off: .50 or less round down, .51 or more round up.
Or, if you know you diff gears, go to: http://www.tommyjeep.com/apps/ratio/index.htm
You can buy the gears you need at your local Jeep or Dodge dealer, Rossers, Parts Mike, or many other off road dealers.
83 CJ8, 4.0 HO/AW4, etc.