Speedometer gear calculator

Ideal driven gear teeth = current teeth × indicated speed ÷ actual speed.

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Current driven gear teeth

Indicated speed

More inputs and assumptions

Measured actual speed

Answer

…

Ideal driven gear teeth = current teeth × indicated speed ÷ actual speed.

More results

Precision and calculation record

Decimal places in numeric results and text downloads

Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

Saved scenario

Save the current answer, then change an input to compare. This clears when you leave.

Formula and assumptions

For a mechanical speedometer driven through a replaceable driven gear, assuming the driven gear’s tooth count inversely scales indicated speed. The rounded result is not a compatibility recommendation. Use safe recorded measurements, not a phone while driving; verify available gears, tooth engagement and the vehicle’s service instructions.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-29

Using Speedometer gear calculator

Ideal driven gear teeth = current teeth × indicated speed ÷ actual speed.

Next: Gear ratio calculator

Next: Tire size calculator

A worked example

For current driven gear teeth = 20, indicated speed = 66, measured actual speed = 60, ideal driven gear teeth = 22.

Before you use the result

Example results for current driven gear teeth

These examples use Indicated speed: 66; Measured actual speed: 60. They are reference calculations, not recommended settings.

Current driven gear teeth	Ideal driven gear teeth
10	11
20	22
40	44

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