If you've made a differential gear swap, wheel swap or tire size swap, the following might help you figure out what gears you need for your speedometer to restore its accuracy. If your speedometer was accurate before the swap then you might be able to get a good estimate by calculating the difference between the old and new ratio and then increasing the driven gear by the same ratio. E.g., going from 2.80:1 to 3.25:1 => 3.25/2.80 = 1.161.
If the old driven gear was 16 teeth then the new should be 16*1.161= ~18.58 (i.e. 18 or 19).
The gears supplied as original equipment are:
Year 
Tire
Size 
Differential Gear 
Driven Gear 
Drive Gear 


Part
Number 
Type 
Color 
Teeth 
Part
Number 
Type 
Color 
Teeth 

'84

225/50/VR16 
3.45 
C0DD17271B 
3a 
Gold
or yellow 
18 
E3ZZ17285B 
4a 
Black 
6 
'85

225/50/VR16 
3.73 
C0DZ17271B 
3a 
Pink 
19 
E3ZZ17285B 
4a 
Black 
6 
'851/2

225/50/VR16 
3.73 
C0DZ17271B 
3a 
Pink 
19 
E3ZZ17285B 
4a 
Black 
6 
'86

225/50/VR16 
3.73 
C0DZ17271B 
3a 
Pink 
19 
E3ZZ17285B 
4a 
Black 
6 











If you are starting from scratch, want a more accurate method for computing speedometer gears, or would like a further understanding of how your speedometer/odometer works the following might help: The speedometer drive system is made up of two gears, the Drive Gear (mounted on the transmission output shaft) and the Driven Gear (mounted on the speedometer cable at the transmission end). The ratio of these two gears determines the final reading on the speedometer and odometer. The goal is to get the speedometer cable to rotate 1000 times per mile. This is done by finding the closest ratio of drive to driven gears that will give you 1000 rotations per mile. The formula that you use to determine this ratio is:
Speedometer Gear Ratio = ((rear axle ratio)*63360)/((Distance car travels with one tire revolution)*1000)
For a spreadsheet that calculates this ratio, and will help you find which gears you need, drop me an email and I'll gladly send it to you – daves@svocop.com
Note that there are two ways to determine the distance of travel with one tire revolution. The easy method (and least accurate) is to measure the radius of the tire, take that measurement and multiply by 2π (or 6.28318). The more accurate way is to perform a rollout of the wheel. To do this, find a helper and measure the distance traveled for one revolution of the tire. Use chalk and mark the ground where you start (use the valve stem as an indicator), drive so that the tire goes one revolution and mark that spot and measure the distance.
Drive gears come in 6, 7, or 8 tooth. Driven gears come in 16, 17, 18, 19, 20 and 21 tooth. The trick to proper gear selection is to determine the ratio you need, then match it up to what Ford offers. The following chart will help you find the closest gear ratio to what you need.
Gear Combination (Driven #/Drive #) 
Ratio 
16/8 
2.0000 
17/8 
2.125 
18/8 
2.25 
16/7 
2.2857 
19/8 
2.375 
17/7 
2.4286 
20/8 
2.5 
18/7 
2.5714 
21/8 
2.625 
16/6 
2.6667 
19/7 
2.7143 
17/6 
2.8333 
20/7 
2.8571 
18/6
or 21/7 
3.0000 
19/6 
3.1667 
20/6 
3.3333 
21/6 
3.5 