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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.

Enter 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.

Frequently asked questions

Why does my speedometer read wrong after changing gears or tires?

A cable-driven speedometer is calibrated for a specific combination of tire revolutions per mile and axle ratio — change either one and the driven gear that used to match no longer does, throwing the reading off.

What if the exact tooth count I need isn't available?

Speedometer driven gears only come in fixed tooth-count increments, so this calculator also shows how far off the nearest available gear will leave your reading — letting you decide whether that small remaining error is acceptable.

Does this work for electronic speedometers too?

No — this is specifically for cable-driven speedometers using a physical driven gear; electronic speedometers are typically recalibrated through a different process (a programmer or a different signal-conditioning approach) rather than swapping a gear.

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