Speedometer Gear Information.
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If you've made a differential gear 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).
For a C4 w/3.25:1 the following applies:
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 determine 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 in inches)*1000)
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 2pi (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 travelled for one revolution of the tire. Use chalk and mark the ground where you start (use the valve stem as an indicator), move the car so that the tire goes exactly one revolution and mark that spot and measure the distance.
Drive gears come in 6 or 7 tooth. Driven gears come in 16, 17, 18, 19, and 20 tooth. There are different drive and driven gears for manual and automatics. Old 3 and 4-speeds are left-hand helix and the speedometer cable enters the transmission on the passenger side. Automatics are right-hand helix and enter the transmission on the driver side.
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.