Free Speedometer Gear Calculator

The Speedometer Gear Calculator is a calibration tool that helps you determine the proper number of teeth on the driven gear when you change tire sizes on your vehicle. Matching the speedometer gear ratio to the new tire diameter ensures that the dashboard speed reading remains accurate. Whether you’re switching to larger all‑terrain tires or smaller performance rubber, this tool lets you compute the exact driven gear teeth required.

How Speedometer Gears Work

A mechanical speedometer relies on a pair of gears: a drive gear attached to the transmission output shaft and a driven gear that turns the speedometer cable and needle. The ratio between these two gears defines the relationship between tire revolutions and the indicated speed. When you alter the tire diameter, the number of revolutions per mile changes, so the gear ratio must be recalculated to keep the speedometer accurate. This is where the Speedometer Drive Gear Calculator — also commonly called a Speedometer Gear Ratio Calculator — becomes essential.

The Calculation Formula

First, determine the number of tire revolutions per mile ($R$) using the tire diameter $D$ in inches:

$$R = \frac{63360}{\pi \times D} \approx \frac{20168}{D}$$

(The constant 20168 comes from 63360 inches per mile divided by π.)

Next, the required number of teeth on the driven gear is given by:

$$\text{Driven Teeth} = \frac{A \times R \times N}{1001}$$

where:

• $A$ = axle ratio (e.g., 3.00, 3.55, 4.10),
• $N$ = number of teeth on the drive gear,
• 1001 is a calibration constant corresponding to a speedometer that reads 1 mile per 1000 cable revolutions, with a small tolerance included.

This formula effectively computes the driven gear teeth needed to maintain the correct speedometer gear ratio for the new tire size.

How to Use the Calculator

1. Enter the tire diameter in inches.
2. The tool automatically calculates the revolutions per mile.
3. Input the axle ratio of your vehicle.
4. Provide the number of teeth on the existing drive gear.
5. The calculator outputs the recommended number of teeth for the driven gear.

The result is normally rounded to the next whole number, but you should also check availability of driven gears with that tooth count, as aftermarket gears often come in specific sizes.

Example Calculation

Suppose you upgrade to tires with a diameter of 20 inches, your axle ratio is 3.00, and your current drive gear has 40 teeth.

• Revolutions per mile: $R = 20168 / 20 = 1008.4$
• Driven teeth = $(3.00 \times 1008.4 \times 40) / 1001 \approx 120.9$, rounded to 121 teeth.

Thus, a driven gear with 121 teeth would be the correct choice to keep your speedometer reading accurately with the 20‑inch tires.

Using a dedicated Tire Size Speedometer Gear calculator like this one eliminates guesswork and helps you avoid costly trial‑and‑error with different gears. Whether you’re recalibrating after a lift kit or simply changing to a different tire profile, the Speedometer Calibration Tool provides a quick, reliable result.