Speedometer gear calculator
Find the theoretical driven gear count for a cable speedometer.
Formula
Tire rev/mi = 1609.344 / (π × diameter in m); driven teeth = drive teeth × axle ratio × tire rev/mi / calibration.
Tire diameter: 28 in · Geometric tire revolutions per mile ≈ 1609.344÷(pi×(28 in × 0.0254 m/in)) · Theoretical driven teeth ≈ (7)×(3.5)×(720.2897996)÷(1,000)
How to use
- Enter the tire diameter and drive gear tooth count.
- Enter the axle ratio and cable revolutions per mile.
- Compare the theoretical count with compatible gears.
Examples
- 28 in, 7 drive teeth, 3.5 axle ratio and 1,000 rev/mi give about 17.647 teeth.
Frequently asked questions
Why is the answer fractional?
Only whole tooth counts exist; choose a compatible available gear and verify the reading.
Is calibration always 1,000?
No. Read the gauge or supplier specification; 1,000 rev/mi is only an example.
Does tire load matter?
Yes. Diameter gives a geometric estimate; load, pressure and wear affect rolling circumference.
Sources
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