The driven gear is located in a removable housing, usually in the tailhousing, of transmissions.
The following formula is very useful when there is no established baseline from which to work, such as replacing a transmission in a vehicle that was purchased without a driveline.
# Drive Teeth x Axle Ratio x Tire Rev. per Mile ÷ 1001 = # Driven Teeth
Tire Rev. per Mile = 20168 ÷ Tire Diameter In Inches