How to Figure Speedo Gear Ratios
- 1). Determine your axle ratio. You do this by finding the number of teeth in your axle ring gear and the number of teeth in the axle pinion gear. Divide the number of teeth in the ring gear by the number of teeth in the pinion gear. For example, if you have 25 teeth in your ring gear and 10 teeth in the pinion gear, your axle ratio will be 2.5:1.
- 2). Determine your tire diameter in inches. You can get this information from your tire manufacturer. A typical tire diameter is 26 inches.
- 3). Determine the speedometer gear ratio by using the formula SG = (20.2 x axle ratio)/TD. Using the examples from steps 1 and 2, the speedometer gear ratio is 1.94 (20.2 x 2.5/26).