Formula for Calculating Speedometer Gears
The following formula will work for any GM, Ford or Chrysler vehicle and is very useful when there is no established baseline from which to work from such as replacing a transmission in a vehicle that was purchased without a drive line. The accompanying charts show what gears were manufactured for each type transmission.
Note: Drive gear refers to the gear that is on the transmission output shaft. The driven gear is located in a removable housing usually in the tailhousing of the transmission. Be aware that some transmissions will have different driven gear housings depending on the tooth count of the driven gear.