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Mechanical Speedometer Gear Ratio at 1001 Revs/Mile

Relate tire diameter or rolling revolutions, axle ratio and drive/driven gear tooth counts using the captured 1001 cable-revolutions-per-mile calibration.

  • Formula & worked example
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

At 800 tire revolutions per mile, a 3.5 axle ratio and a 10-tooth drive gear require an ideal driven ratio of about 27.972 teeth under the 1001-revolutions-per-mile calibration.

Tire rev/mile = 63360 / (π × diameter in inches); driven teeth = drive teeth × axle ratio × tire rev/mile / 1001

Worked example

Enter these known values and leave the other values blank.

Diameter of tire (D)
635 mm
Axle ratio (A)
3.5
Drive gear tooth count / ideal ratio
10
Tire rolling revolutions per selected distance
501.3 per km
Ideal driven gear tooth ratio
28

Assumptions and limitations

  • The retained calibration is 1001 cable revolutions per mile. Confirm the instrument’s calibration before applying this model; it is not a universal speedometer constant.
  • Diameter gives an ideal unloaded circular circumference. Real rolling circumference depends on tire construction, load, wear and inflation; measured revolutions per distance can be entered instead.
  • Either gear field can represent a fractional ideal ratio when solved backward. Actual gears have whole teeth; enter their actual counts for forward calculations. Selecting an available gear changes the indicated speed and requires a separate calibration check.
  • This computes an ideal ratio, not an installation or road-speed certification. Mechanical compatibility and actual indicated speed must be checked separately.

Common questions

Why can the result have a fraction of a tooth?

It is the exact ratio required by the supplied dimensions and fixed calibration. A physical gear must have a whole tooth count, so rounding changes the calibration.

References

Bookify derives tire revolutions from exactly 63360 inches per mile and pi instead of the rounded 20168 constant, retaining the captured 1001-revolutions-per-mile instrument calibration.

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