Speedometer gears govern how accurately a speedometer tracks speed. The gears depend on the tire size and axle gear configuration. Axle gear configuration depends on the number of teeth in the rear and pinion gear. In fact, to figure the correct speedometer gear ratio, you will need to know the axle gears and the tire diameter. Once you determine the speedometer gear ratio, you can choose the proper set of speedometer gears to reach this ratio.
Calculate the axle ratio. Divide the number of teeth on your axle ring gear by the number of teeth on your axle pinion gear. As an example, if you have 20 teeth in your ring gear and 10 teeth in the pinion gear, your axle ratio will be 2.0:1: 20/10.
Measure or find your tire diameter in inches. You can measure this or secure it from your tire manufacturer by specifying your make and model. As an example, use 24 inches.
Calculate the speedometer gear. Use the formula: SG = (20.2 x axle ratio)/TD where TD is the tire diameter. Using the above examples, the speedometer gear ratio is 1.68: 20.2 x 2.0/24.
Determine the speedometer gears by matching the speedometer gear teeth necessary to reach the speedometer gear ratio. For example, if you choose 25 teeth in your top gear, calculate the number of teeth necessary in your bottom gear to achieve a gear ratio of 1.68 using the formula: Bottom Gear = Top Gear/Gear Ratio. Using the numbers from the example above, your bottom gear will have 15 teeth: 25/1.68. If you know the bottom gear teeth and wish to determine the top gear teeth, use the formula: Top Gear = Gear Ratio x Bottom Gear.
