Electronic speedometers and engine computers count pulses from a speed sensor on the transmission output or differential. After changing tyre size or axle gears, the PPM must be recalibrated or the speedometer and odometer will read wrong.
How to use the Pulse per Mile Calculator
- Tyre diameter
- Axle ratio
- Reluctor teeth
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the tyre’s overall diameter, the axle (final-drive) ratio and the number of teeth on the reluctor ring.
Inputs explained
| Field | What to enter |
|---|---|
| Tyre diameter | Units: in, cm, mm, ft, m |
| Axle ratio | A number |
| Reluctor teeth | A number |
What each result means
- Wheel revolutions per mile = 1609.344÷(pi×[Tyre diameter])
- Pulses per mile = [Wheel revolutions per mile]×[Axle ratio]×[Reluctor teeth]
- Pulses per kilometre = [Pulses per mile]÷1.609344
Example values
How the main result changes when Tyre diameter changes and everything else stays at the example values above:
| Tyre diameter (in) | Pulses per mile |
|---|---|
| 14 | 214,934 |
| 21 | 143,290 |
| 28 | 107,467 |
| 35 | 85,974 |
| 42 | 71,645 |
| 56 | 53,734 |
Formula
Wheel revolutions per mile = 63,360 ÷ (π × tyre diameter in inches)
PPM = revolutions per mile × axle ratio × reluctor teeth
Worked example
Inputs: Tyre diameter: 28 in; Axle ratio: 3.73; Reluctor teeth: 40.
Results: Wheel revolutions per mile = 720.3; Pulses per mile = 107,467; Pulses per kilometre = 66,777.
Frequently asked questions
Why recalibrate after a tyre change?
Bigger tyres turn fewer times per mile, so the speedometer reads low unless the PPM is updated.
What is the standard PPM?
Many GM vehicles use 4,000 PPM at the output; other makes use different values.
Where do I find the reluctor teeth count?
In the service manual, or by counting teeth on the tone ring.