Speedometer Gear Reducer

[69 Post Sedan]

Ths is the formula I used.

Kurt

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">First off, the way to get the whole system to work right is to make the speed cable rotate 1,000 revolutions for every mile the car is driven. This is done by adjusting the transmission driven to drive gear ratio on the speed cable to match the tire size and rear axle gear ratio of the car.

The first thing to know is how many revolutions the tires make in one mile. To calculate this, measure the diameter of your tires. 26&quot; is about average for F60-15's or 245-60-15's. Multiply this measurement (26&quot by pi (3.1416). This will give you the circumference of the tire. Next, divide one mile in inches (63360) by the circumference of tire (63360 divided by (26 x pi)=776). This equals the revs per mile of the tire. Now multiply this number by the rear axle ratio to give you the number of drive shaft revs per mile (776 x 3.91= 3033). Next, divide this number by 1000 (3033 divided by 1000 = 3.033). This is the ratio of transmission driven/drive gears on speedometer cable you will need in this case use a driven gear with 18 teeth and a drive gear with 6 teeth (18 divided by 6 = 3.0). This is the best choice which yields a 1.1% high error which means your odometer unit indicates 1.1% greater than actual or 10.11 for an actual 10.0 miles. If you are using the Ford adapter/reducer (reducer was factory equipped on cars with 3.91, 4.11 or 4.30 rear axle ratios), multiply the number by 0.75 (3.033 x 0.75=2.274). This is the driven/drive ratio you need. A 3.75% error (i.e., a 0.375 mile error in 10 miles) is considered acceptable by design.</div></div>