Speedometer Gear Ratio Calculator

About the Speedometer Gear Ratio Calculator

Changing tire sizes, axle ratios, or transmissions throws off your speedometer accuracy. A factory speedometer is calibrated to a specific combination of tire revolutions per mile, axle ratio, and transmission output gear. Alter any of these, and your speedometer will read too fast or too slow — which can lead to speeding tickets, incorrect odometer readings, and warranty disputes.

Our Speedometer Gear Ratio Calculator determines the exact driven gear tooth count needed to restore speedometer accuracy after modifications. Enter your tire size, axle ratio, transfer case ratio (if applicable), and transmission type, and the calculator computes the required driven gear based on the standard formula used by GM, Ford, and Chrysler cable-driven and electronic speedometers.

This calculator is essential for hot rodders, off-road enthusiasts, and anyone who's swapped tires or differentials. It shows the theoretical vs. actual speed error, recommended gear tooth counts, and whether you need a speedometer gear adapter for tooth counts outside the stock range.

When This Page Helps

Use this calculator after a tire, axle, or transmission change when the speedometer no longer matches reality. It helps you narrow down the right driven gear or correction strategy before you start ordering parts or chasing an odometer error by trial and error. That saves time when the correction is a gear swap instead of a full gauge replacement.

How to Use the Inputs

1. Enter your tire diameter in inches or select tire size (e.g., 275/60R15).
2. Input your rear axle (differential) ratio (e.g., 3.73).
3. Enter the number of drive gear teeth on your transmission (usually 7 or 8).
4. Optionally enter a transfer case ratio for 4WD vehicles.
5. Review the calculated driven gear tooth count and speed error.
6. Check the gear availability table for common tooth counts.
7. Order the recommended driven gear to correct your speedometer.

Example Calculation

Tips & Best Practices

• Always measure actual tire diameter under load, not sidewall specs.
• Drive the vehicle with GPS to verify actual speed before and after.
• Speedometer gears are color-coded by tooth count — note the color when ordering.
• If between gear sizes, choose the one that makes the speedometer read slightly fast (safer).
• Check that your driven gear housing angle matches the transmission — some are 90°.
• For electronic speedometers, consider a SpeedCal or Dakota Digital signal conditioner.

How Speedometer Gears Work

In a cable-driven speedometer, a drive gear on the transmission output shaft spins a driven gear in the tailshaft housing. The driven gear connects via a cable to the speedometer head. The ratio between drive and driven gear teeth, combined with tire revolutions per mile and axle ratio, determines the displayed speed.

Common Tooth Count Ranges

GM TH350/TH400: drive gears come in 7, 8, or 9 teeth; driven gears range from 18-45 teeth. Ford C4/C6: drive gears are 7 or 8 teeth; driven gears 16-23 teeth. Chrysler 727/904: drive gears are typically 8 teeth; driven gears 21-39 teeth.

Electronic Speedometer Calibration

Modern vehicles use a Vehicle Speed Sensor (VSS) that outputs a set number of pulses per mile. When you change tire size or axle ratio, you can either swap the physical gear (if applicable) or install an electronic speed signal calibrator that multiplies or divides the pulse count to correct the reading.

Frequently Asked Questions

• What is a speedometer driven gear?

  The driven gear is a plastic or nylon gear in the transmission tailshaft that meshes with the drive gear on the output shaft. Its tooth count determines the speedometer signal rate. That is the part you usually swap to correct the reading.

• How do I find my current drive gear teeth?

  The drive gear is on the transmission output shaft. Check your transmission's service manual or count the teeth by removing the driven gear housing. Some transmissions also list the tooth count on the gear itself.

• What if the calculated tooth count isn't available?

  If the exact tooth count isn't made, choose the closest available. For cable speedometers, you may need a gear adapter or ratio adapter box. The goal is to get the error close enough that the dash reading stays usable. Small differences are usually less important than being in the correct range.

• Does this work for electronic speedometers?

  Electronic speedometers use a vehicle speed sensor (VSS) that produces pulses per mile. The same formula applies, but correction is done via an electronic signal calibrator instead of gear swaps. It is the same calibration problem with a different hardware fix. That makes the calculator useful even when the hardware is different.

• How accurate does the speedometer need to be?

  Most states allow ±5% error. Federal standards require speedometers to never read lower than actual speed — reading slightly fast is acceptable. That is why a small fast-reading bias is usually safer than the other way around.

• What if I have a transfer case?

  Enter your transfer case ratio (e.g., 2.72 for NP205 low range). In high range, the ratio is typically 1:1. Low range is only for off-road use, so calibrate with the range you drive on-road.