Speedometer Gear Calculator

To find the correct speedometer gear, the calculator first determines the tire revolutions per mile based on the overall outer tire diameter. The primary formula used for tire revolutions is: Revolutions = 20,168 / Diameter (in inches). Once the revolutions per mile are established, the required number of teeth on the driven gear is calculated using the equation: Driven Gear = (Driving Gear × Axle Ratio × Revolutions) / 1,001. This mathematical relationship ensures that the speedometer cable rotates at the precise speed required to match your new tire and differential setup.

Imagine you are upgrading your classic truck with larger tires and a new differential. Your specifications are as follows: Tire Diameter = 30 inches, Axle Ratio = 3.73, and your transmission has a Driving Gear with 8 teeth. First, calculate the tire revolutions per mile: Revolutions = 20,168 / 30 = 672.27 revolutions per mile. Next, plug this value into the driven gear formula: Driven Gear = (8 × 3.73 × 672.27) / 1,001. This simplifies to Driven Gear = 20,051.15 / 1,001, which equals approximately 20.03. Rounding to the nearest whole number gives you a 20-tooth driven gear.

Always measure your actual loaded tire diameter with a tape measure on the ground rather than relying solely on the sidewall manufacturing numbers, as vehicle weight compresses the tire. Verify your current transmission driving gear tooth count before ordering replacement parts, as these come in both permanent shaft-mounted versions and removable slip-on styles. Remember that slight variations in rounding may require you to choose between two adjacent gear sizes; select the one closest to your decimal result for optimal accuracy.