Install bigger tires or swap your rear axle ratio, and your speedometer will start lying to you, because the driven gear that translates transmission output into a speedometer reading was calibrated for your old setup. This calculator finds the correct replacement tooth count so your speedometer reads accurately again.
How to use this speedometer gear calculator
- Enter your tire’s overall rolling Diameter (not the wheel/rim size) and its unit.
- Enter your rear Axle Ratio (e.g. 3.55 for a 3.55:1 ratio).
- Enter the tooth count of your existing Drive Gear (the one on the transmission output shaft, usually unchanged).
- Read the calculated Driven Gear tooth count you need.
What this speedometer gear calculator does
Your speedometer reads speed based on how fast a drive gear in the transmission spins, converted through a driven gear to turn the speedometer cable (or sensor) at the correct rate. That conversion assumes a specific tire diameter and rear axle ratio, change either one and the wheel now turns a different number of times per mile than the system was calibrated for, so the old driven gear produces an inaccurate reading unless it’s swapped for one with the correct tooth count.
Revolutions per Mile (R) = 20,168 ÷ Tire Diameter (inches)
Driven Gear Teeth = Axle Ratio × R × Drive Gear Teeth ÷ 1001
Why the 1001 constant appears in the formula
The 1001 constant, along with the 20,168 figure used to derive revolutions per mile from tire diameter, are standard values from the traditional mechanical speedometer gear formula used across the automotive industry for decades, encoding the fixed relationship between speedometer cable rotation and vehicle speed for a given final drive setup.
Why your result gets rounded
Physical driven gears are only manufactured in whole-tooth increments, so the calculated exact value is rounded to the nearest tooth count actually available as a part. This means your speedometer may read slightly off even with the closest available gear, a small margin is normal and expected. After installing the new gear, verify your actual speedometer accuracy against a GPS-based speedometer app to confirm the real-world result.
Frequently asked questions
Why does changing my tire size affect my speedometer?
A bigger or smaller tire changes how many times the wheel rotates per mile traveled, but the speedometer gearing was calibrated for your original tire’s rolling diameter, so without recalibrating the driven gear, the speedometer reading no longer matches your actual speed.
Why is my calculated result rounded?
Driven gears are only manufactured in whole-tooth increments, so the exact calculated value is rounded to the nearest available part. This can leave your speedometer slightly off even with the correct replacement gear, verify with a GPS speedometer app after installation.
Does changing my drive gear teeth also matter?
Typically the drive gear (on the transmission output shaft) stays the same when only tires or axle ratio change, it’s an input to the formula rather than something you need to swap, unless you’re also changing the transmission itself.
