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Speedometer drive gear calculator

For cable-driven speedometers: the driven gear tooth count that makes your speedometer read true after a gear or tire change, and how far off the nearest available gear will be.

Free, no sign-upUpdated September 2026
Your numbers
Setup
Measure the loaded height for accuracy.
The gear on the transmission output shaft.

Driven gear needed

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Exact tooth count for a true reading.
Use this gear
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Your speedometer will read
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Tire revolutions per mile
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Check the gear's range

Driven gears come in ranges that fit specific housings. If the count you need falls outside your housing's range, you may need a different drive gear or a ratio adapter.

How the gear is calculated

Driven teeth = tire revolutions per mile × axle ratio × drive gear teeth ÷ speedometer turns per mile

A cable-driven speedometer expects its cable to turn a set number of times per mile: 1,001 for most GM units and 1,000 for Ford. The driveshaft turns tire revolutions × axle ratio times per mile, and the drive and driven gears step that down to what the speedometer expects.

Gears only come in whole teeth, so the nearest gear is rarely perfect. The result shows how far off it will read so you can choose between the two closest gears.