Pulse per Mile Calculator (speedometer calibration from tyre size & sensor)

Understanding the pulses per mile calculator — concept and principle

This calculator works out the pulses-per-mile (PPM) setting an electronic speedometer needs, from the tyre diameter, where the speed sensor reads, the axle ratio and the sensor's pulses per revolution. It is used when fitting an aftermarket speedometer or after changing tyre size or axle gears.

Tyre turns × axle × pulses: with the default inputs the result is 21,493 PPM (≈ 13,355 pulses per km).

How it is calculated

Tyre circumference = π × diameter (inches converted with 25.4 mm).
Tyre revolutions per mile = 1,609,344 ÷ (π × D in mm).
Sensor before the axle (transmission or driveshaft): PPM = tyre revolutions × axle ratio × pulses per revolution.
Sensor at the wheel or hub: PPM = tyre revolutions × pulses per revolution.
Pulses per km = PPM ÷ 1.609344.

The principle

An electronic speedometer counts pulses from a sensor and converts them to distance using the PPM setting. Every turn of the tyre moves the vehicle one circumference, so revolutions per mile are fixed by the tyre's size. A sensor on the driveshaft or transmission output turns faster than the wheels by the axle ratio, so it produces more pulses per mile. Bigger tyres mean fewer revolutions per mile; if the PPM is not changed, the speedometer reads low.

Worked example

Default inputs: 28.0 in tyre, sensor on the transmission output, axle ratio 3.73, 8 pulses per revolution.

Diameter = 28 × 25.4 = 711.2 mm; circumference = 2,234 mm.
Tyre revolutions per mile = 1,609,344 ÷ 2,234.3 = 720.3 (447.6 per km).
PPM = 720.3 × 3.73 × 8 = 21,493.
Pulses per km = 21,493 ÷ 1.609 = 13,355.

Each point is the calculator re-run with only “Tyre overall diameter” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Tyre overall diameter	Pulses per mile (PPM)
7	85,974
14	42,987
21	28,658
28	21,493
35	17,195
42	14,329
56	10,747

How to read the result

Enter the PPM in the speedometer's calibration menu, then check against a GPS over a measured distance. A loaded tyre rolls on a slightly smaller radius than its free diameter, so a small correction is often needed. After changing tyre size or axle gears, always recalculate.

Common mistakes

Using the tyre's nominal size instead of its real diameter.
Forgetting the axle ratio when the sensor is on the driveshaft.
Using the wrong pulses per revolution for the sensor.
Mixing pulses per mile and pulses per kilometre settings.

Keep in mind: An inaccurate speedometer can lead to speeding. Verify the calibration with GPS before relying on it.

Frequently asked questions

How do I calculate pulses per mile?

Tyre revolutions per mile × axle ratio (if the sensor is before the axle) × pulses per revolution.

Why does my speedometer read low with bigger tyres?

Bigger tyres turn fewer times per mile, so the sensor sends fewer pulses for the same distance.

How many times does a 28 in tyre turn per mile?

About 720 times.

Sources: NIST Special Publication 811 (2008), Appendix B (1 mile = 1,609.344 m; 1 in = 25.4 mm)

Checked 9 Nov 2026. Rolling-wheel kinematics (distance per revolution = π × diameter) and gear reduction through the axle ratio; the sensor pulse count, axle ratio and tyre diameter are example values. 1 mile = 1,609.344 m; 1 in = 25.4 mm exactly.

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