Governor Gear Calculator: Sizing & Ratio Explained

This governor gear calculator works out the correct gear ratio needed to make a vehicle’s speedometer read accurately after a tire size or differential gear change. You enter your original tire diameter, new tire diameter, and your axle ratio (or the speedometer gear tooth counts you already have), and it returns the required driven gear tooth count and the resulting speedometer error percentage. The calculator below uses the standard formula that links tire circumference, axle ratio, and transmission drive gear teeth to the speedometer cable revolutions.

Formula

The core formula for governor gear sizing is:

Required Driven Gear Teeth = Drive Gear Teeth × (New Tire Diameter ÷ Original Tire Diameter) × (New Axle Ratio ÷ Original Axle Ratio)

Where:

• Drive Gear Teeth – the number of teeth on the transmission output shaft gear that spins the speedometer cable (often 15, 16, or 17).
• Original Tire Diameter – the diameter of the tires the speedometer was calibrated for (in inches or mm).
• New Tire Diameter – the diameter of the tires you are installing.
• Axle Ratio – the differential gear ratio (e.g., 3.73:1). If unchanged, use the same value for both old and new.

If you are only changing tires, the axle ratio cancels out, and the formula simplifies to: New Driven = Old Driven × (New Tire Diameter ÷ Old Tire Diameter).

Worked Example

Using the calculator’s defaults: original tire diameter = 30 inches, new tire diameter = 33 inches, axle ratio = 3.73, and drive gear teeth = 15.

1. Calculate the tire diameter ratio: 33 ÷ 30 = 1.10.
2. Since the axle ratio is unchanged, it cancels out (ratio = 1.0).
3. Multiply the drive gear teeth by the tire ratio: 15 × 1.10 = 16.5.

So you need a driven gear with approximately 16.5 teeth. Since gears come in whole numbers, you would choose a 17-tooth gear, which will make the speedometer read about 3% slow (because the cable will spin slightly slower than ideal). The calculator shows 16.5 teeth and a speedometer error of +10% (meaning the speedometer reads 10% lower than actual speed if you kept the original gear).

How Governor Gears Work

In older vehicles (roughly pre-1990s, but also many trucks and SUVs into the 2000s), the speedometer is driven by a cable connected to the transmission. Inside the transmission, a small plastic or metal gear (the drive gear) meshes with another gear (the driven gear) that turns the cable. The ratio between these two gears is set at the factory to match the original tire size and axle ratio. When you change tire diameter or axle ratio, the cable speed changes, and the speedometer reads incorrectly. The governor gear (also called a speedometer gear) is the driven gear you swap to restore accuracy.

Modern vehicles use electronic speed sensors, but the same math applies to recalibrating the PCM or using a tuner. The physical gear swap is still common in classic cars, off-road rigs, and trucks.

How to Choose the Right Gear

To select the correct governor gear, you need to know three things:

• Your original tire diameter (the size the speedometer was calibrated for – often listed on the door jamb sticker).
• Your new tire diameter (measure the actual installed height, not the sidewall stamp).
• Your axle ratio (if changed).

Then use the formula above. The result is the number of teeth on the driven gear you should install. Gears are available in various tooth counts (typically 15 to 45 teeth) from aftermarket suppliers. Choose the closest whole number – if the result is between two sizes, pick the one that gives the smallest error. For example, if the formula says 18.2, a 18-tooth gear will make the speedometer read slightly fast (by about 1.1%), while a 19-tooth gear will make it read slightly slow (by about 4.4%). The 18-tooth gear is usually the better choice.

Common Mistakes and Misconceptions

A frequent error is using the tire size stamped on the sidewall (e.g., 33×12.50R15) as the actual diameter. The true diameter varies with air pressure, load, and tread wear. Always measure the tire mounted on the vehicle, inflated to normal pressure, and sitting on the ground. Another mistake is forgetting that changing the axle ratio requires a different gear even if the tires are unchanged. Also, some people assume that a larger tire always makes the speedometer read slow – that is true, but only if the axle ratio stays the same. If you regear to compensate (e.g., from 3.73 to 4.10), the two changes can cancel out or even overshoot.

A common misconception is that the governor gear is located in the differential. It is actually in the transmission tailshaft or transfer case (in 4x4s). Also, the drive gear is not always accessible without disassembling the transmission; in some cases, you must change both gears.

When to Use This Calculator

Use this calculator whenever you change tire size or axle ratio and you have a cable-driven speedometer. It is also useful for planning a gear change – you can calculate what driven gear you will need before you buy parts. The same math applies to electronic speedometer recalibration, but instead of swapping gears, you would enter the new gear ratio into a tuner or programmer. The calculator is also handy for verifying that a used vehicle’s speedometer is accurate after modifications.

Limitations and Real-World Considerations

The formula assumes a linear relationship, which is accurate for cable-driven speedometers. However, it does not account for:

• Tire growth at high speed (centrifugal force can increase effective diameter slightly).
• Wear on the gears – a worn gear can slip or skip, causing erratic readings.
• Aftermarket speedometer calibrators that adjust electronically – those bypass the mechanical gear entirely.

Also, the formula gives a fractional tooth count, but gears are only available in whole numbers. The resulting speedometer error is usually under 5%, which is acceptable for most driving. For precise calibration, you can use a GPS speedometer app to verify the actual error and then adjust the gear if needed.

Frequently Asked Questions

What is a governor gear?
A governor gear is a small gear in the transmission that drives the speedometer cable. It is also called a speedometer gear or driven gear. It works with the drive gear to convert transmission output speed to a cable speed that matches the vehicle’s tire and axle combination.

How do I know if my speedometer gear is wrong?
If your speedometer reads too high or too low compared to a GPS or a known-accurate vehicle, the gear ratio is likely incorrect. This often happens after a tire or gear change. You can also check the speedometer error by driving at a steady speed and comparing the reading to a GPS app.

Can I use this calculator for metric tire sizes?
Yes, as long as you use the same unit for both old and new tire diameters. The formula is unit-agnostic – it uses the ratio, so inches, millimeters, or centimeters all work. Just be consistent.

What if I changed both tires and axle ratio?
You need to know the original axle ratio (before the change) and the new axle ratio. Enter the original ratio in the “original axle ratio� field and the new ratio in the “new axle ratio� field. The calculator will combine both effects.

Where can I buy a governor gear?
Aftermarket speedometer gears are available from auto parts stores, off-road suppliers, and online retailers. You need to know the transmission model (e.g., TH350, 700R4, NP205) and the shaft size. Many suppliers list gears by tooth count and transmission type.

Is there an alternative to changing the gear?
Yes. You can use an electronic speedometer calibrator that installs in the cable or uses the vehicle’s sensor signal. These devices adjust the signal electronically and can be tuned to any ratio. They are more expensive than a gear but allow fine adjustment and are reusable.

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in old_tire | Original tire diameter | in | 30 | 10
in new_tire | New tire diameter | in | 33 | 10
in axle | Axle ratio (e.g. 3.73) | - | 3.73 | 1
in drive | Drive gear teeth | teeth | 15 | 1
out gear | Required driven gear teeth | teeth | round(drive * (new_tire / old_tire) * (axle / 3.73), 1) | 1
out error | Speedometer error | % | ((new_tire - old_tire) / old_tire) * 100 | 1