Speedometer Gear
Calculate the correct driven gear teeth count for your vehicle speedometer after axle ratio, transmission, or tire size changes.
How to Calibrate Your Speedometer
When you make modifications to your vehicle, such as installing larger off-road tires, changing the rear-end gear ratio, or swapping the transmission, your speedometer reading will become inaccurate. In older mechanical speedometer systems, this is corrected by replacing the speedometer driven gear at the transmission housing.
This Speedometer Gear Calculator determines the exact and rounded number of teeth required on your speedometer driven gear to recalibrate your speedometer back to factory accuracy.
The Speedometer Driven Gear Formula
The required driven gear teeth count is calculated using the following mechanical formula:
Driven Gear Teeth = (Drive Gear Teeth * Axle Ratio * Tire Revs per Mile) / Calibration Constant
- Drive Gear Teeth: The number of teeth on the gear attached to the transmission output shaft (typically 6, 7, 8, or 9 teeth).
- Axle Ratio: The gear ratio of your vehicle's rear differential (e.g., 3.73, 4.10, 4.56).
- Tire Revolutions per Mile: How many times your tire rolls in a single mile, calculated as
20,168 / Tire Diameter (inches). - Calibration Constant: The number of cable revolutions per mile for your speedometer head. The domestic standard for GM, Ford, and Chrysler speedometers is 1001.
Determining Metric Tire Diameter
If your tire size is metric (e.g., 225/60/R15), its overall diameter is computed using:
Tire Diameter = 2 * (Width * Aspect Ratio / 2540) + Wheel Size
Once you calculate the required number of teeth, you should round it to the nearest whole tooth and purchase the corresponding color-coded gear for your specific transmission type.
Frequently Asked Questions
What is the difference between drive gear and driven gear?
The drive gear is the gear installed internally on the transmission's output shaft. The driven gear is the smaller, easily replaceable gear attached to the end of the speedometer cable housing that meshes with the drive gear.
How does tire size affect my speedometer?
Larger tires have a larger circumference, meaning they travel further with each revolution. This causes your speedometer to read slower than your actual vehicle speed. Conversely, smaller tires will make your speedometer read faster than your actual speed.
What is the standard constant for domestic speedometers?
Most American domestic vehicles (GM, Ford, Mopar) use a standard speedometer calibration constant of 1,001 cable revolutions per mile at 60 mph. Some imports or specialized speedometers might use 1,000 or other values.
How do I know my vehicle's rear axle ratio?
The axle ratio is often stamped on a metal tag attached to one of the differential cover bolts. Alternatively, you can look up the axle code on your vehicle's door jamb safety certification label, or calculate it by counting driveshaft rotations relative to one full wheel rotation.
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