To calculate how many teeth your speedometer gear should have, use the following formulas. First calculate your tire’s circumference. This is easiest to do by just measuring it with a tape measure.
Then to get tire revolutions per mile, divide 20168 by the tires circumference. Use this calculation in the formula below.
The number of drive teeth refers to the gear that is on the transmission output shaft located in a removable housing usually in the tail housing of the transmission. It will usually be either 6, 7 8, or 9 teeth for fords.
The axle ratio used in the formula can be determined by marking a line on your driveshaft and tires. Then turn your drive shaft however many times until your tires turn one complete rotation. if your driveshaft turns 3 times, and your tires turn once, you have a 3.00:1ratio. Use 3 in the axle ratio. If the drive shaft turns 3 1/2 times for your tire to turn once, use 3.5 etc.
# of drive teeth x axle ratio x tire revolutions per mile
The answer to the above formula is the number of “Driven teeth” you need on your speedometer cable.