Speedometer Gear Calculator
When you change your tire size or axle ratio, your mechanical speedometer reads incorrectly because the driven gear inside the transmission or tail housing no longer matches the new setup. Enter your drive gear teeth, axle ratio, and tire diameter (or P-metric size) to find the exact driven gear you need and see how much speedometer error your current gear is causing.
Why your speedometer reads wrong after a tire or gear change
A mechanical speedometer counts cable revolutions driven by a small plastic or nylon gear on the transmission output shaft. That drive gear meshes with a driven gear inside the speedometer cable housing or the tail housing. The ratio between those two gears, combined with the axle ratio and tire circumference, determines how many cable turns equal one mile per hour. When you fit a larger tire, the tire travels further per rotation so the cable turns fewer times for a given road distance, making the speedometer read low. A shorter tire does the opposite. A numerically higher axle ratio (more aggressive gearing) spins the output shaft faster relative to road speed, making the speedometer read high. Changing either one knocks the original calibration out of balance, and the only mechanical fix is to swap the driven gear to a tooth count that restores the correct ratio.
The speedometer gear formula
The number of driven gear teeth required is: driven = (drive teeth) x (axle ratio) x (revolutions per mile) / 1,001. The revolutions per mile figure comes from the tire diameter in inches: rev/mile = 20,168 / diameter. The constant 1,001 is the number of drive gear rotations per mile that GM, Ford, and Chrysler transmissions were engineered to produce per 1,000 output shaft revolutions (the actual engineering constant varies slightly by transmission family, but 1,001 is the widely accepted standard for most applications). Because gears come only in whole tooth counts, you round the exact result to the nearest integer and accept a small rounding error. An error below 2% is considered excellent. Most gear suppliers stock driven gears in every tooth count from about 15 to 45 teeth, identified by color, so once you know the tooth count you order the matching color-coded gear for your transmission tail housing.
How to find your drive gear tooth count and axle ratio
The drive gear is mounted on the output shaft inside the transmission tail housing or transfer case. On most GM TH350, TH400, and 700R4 transmissions it is accessible by removing a single bolt and the speedometer cable retainer. Common drive gear counts are 6, 7, or 8 teeth. For Ford C4, C6, and AOD units it is usually 7 or 8. Chrysler Torqueflite (727/904) typically uses a 6 or 7 tooth drive gear. Your axle ratio is stamped on a tag on the axle housing or differential cover, printed in your vehicle build sheet, or found in the factory service manual. Aftermarket axles should have documentation from the manufacturer. Tire diameter can be read from a P-metric tire size using the calculator in P-metric mode, or measured directly on the vehicle with a tape measure from the ground to the top of the inflated tire, multiplied by 2.
Electronic speedometers and modern vehicles
This calculator is designed for vehicles with mechanical cable-driven speedometers, which were standard on most passenger cars and trucks through the mid 1990s. Vehicles with electronic speedometers (a Vehicle Speed Sensor sending a digital pulse to the instrument cluster) require a different approach: either reprogramming the PCM with a tuner, installing an electronic speed signal corrector, or on some platforms changing the pulse-per-mile setting in the ECU. If you have an electronic speedometer and no mechanical cable, this gear calculator does not apply to your vehicle.
Speedometer error by driven gear accuracy
| Error range | Rating | Effect at 60 mph |
|---|---|---|
| 0 - 2% | Excellent | Within 1.2 mph |
| 2 - 5% | Acceptable | 1.2 - 3.0 mph off |
| 5 - 10% | Poor | 3 - 6 mph off |
| Over 10% | Unacceptable | Over 6 mph off |
Accuracy benchmarks used by most transmission suppliers when selecting speedometer gears.
Frequently asked questions
What is the 1,001 constant in the speedometer gear formula?
The 1,001 constant represents the number of drive gear rotations per mile for a properly calibrated speedometer in most domestic transmissions. It is derived from the original GM engineering specification that defined 1,000 driven gear teeth per mile as the target, with a slight correction factor. Using 1,001 in the denominator gives accurate results across GM, Ford, and Chrysler mechanical-cable setups. Some specialty or import applications use a different constant, so always verify your results with a GPS after installation.
How do I find the overall diameter of my P-metric tire?
A P-metric tire size like 265/70R17 tells you everything you need. The first number is the tread width in millimetres (265). The second number is the aspect ratio as a percentage (70), meaning the sidewall height is 70% of 265 mm. The last number is the wheel rim diameter in inches (17). To get the overall diameter: sidewall height = 265 mm x 0.70 = 185.5 mm = 7.303 inches. Overall diameter = 17 in + 2 x 7.303 in = 31.6 in. Switch this calculator to P-metric mode and it does all of this automatically.
How accurate does my speedometer need to be?
Most transmission suppliers rate anything under 2% as excellent and 2-5% as acceptable. A 5% error at 60 mph means your speedometer reads 3 mph off. In practice this matters for avoiding speed camera or officer citations, where reading slightly high (you think you are going 63 but actually doing 60) is safer than reading low. Aim for the smallest possible error, but if the closest available gear only gets you to 3%, that is generally fine for street use. Verify with a GPS-based speed app after installation.
Can I change just the driven gear, or do I need to change the drive gear too?
In almost all cases, changing only the driven gear is sufficient. The drive gear is pressed onto the output shaft and comes in only a few tooth counts per transmission family. Driven gears are the variable element, available in a wide range of tooth counts (typically 17-45 for most GM, Ford, and Chrysler housings) and are cheap and easy to swap. Only if the required driven gear tooth count falls outside the available range (very large tires or very unusual axle ratios) would you consider a different drive gear.
What do the driven gear colors mean?
Most manufacturers color-code driven gears so you can quickly identify tooth count without counting. The color system varies by transmission family. For example, GM 700R4/4L60 gears run from purple (17 teeth) through white, yellow, orange, green, and so on up to about 45 teeth. The color chart is specific to each transmission type, so once you know the correct tooth count, look up the matching color for your particular transmission and order accordingly. Most speed shops and online suppliers list the color alongside the tooth count.
Does a tire size change affect my odometer as well?
Yes. A mechanical odometer is also driven by the speedometer cable, so it is subject to the same error. If your speedometer reads 3% high because of a tire change, your odometer is also accumulating miles 3% faster than actual road distance. This can affect fuel economy calculations and service interval tracking, and it shows up at resale as slightly inflated mileage. Installing the correct driven gear fixes both the speedometer and the odometer at the same time.