How this calculator works
A vehicle speed sensor counts pulses from a toothed reluctor as the wheel, transmission output shaft or differential turns. If the reluctor is on the axle, the pulse count per mile is the number of wheel revolutions per mile multiplied by the axle ratio and the number of teeth. Wheel revolutions per mile come from the tire circumference. A larger tire covers more ground per revolution, so it produces fewer pulses per mile for the same gearing. The result is used when calibrating a speedometer, changing tire size or programming an engine control unit.
Formula
Wheel revolutions per mile = 1,609.344 m / (pi x tire diameter in m). Pulses per mile = wheel revolutions per mile x axle ratio x reluctor teeth. Pulses per kilometre = pulses per mile / 1.609344.
Worked example
A 700 mm tire has a circumference of 2.199 m, so it turns 731.9 times per mile. With a 3.73 axle ratio and 40 reluctor teeth the sensor produces 731.9 x 3.73 x 40 = 109,200 pulses per mile, or about 67,854 pulses per kilometre.
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Frequently asked questions
Where is the reluctor usually fitted?
It can be on the transmission output shaft, the differential pinion, the axle shaft or a wheel hub. The formula here assumes the pulses are multiplied by the axle ratio, which is true for a reluctor before the final drive. If your sensor is after the differential, leave the axle ratio out.
Why does a tire change affect the speedometer?
A larger diameter covers more distance per revolution, so the vehicle travels farther than the speedometer expects for the same pulse count. The indicated speed then reads lower than the actual speed.
What is a reluctor tooth count?
It is the number of teeth or windows on the tone ring that the sensor counts per revolution. Common values are 40, 44 and 48. Count the teeth or look up the part for your transmission or axle.
Does tire pressure change the result?
Yes, slightly. A tire running at low pressure has a smaller effective rolling radius, which changes the revolutions per mile. Use the loaded radius or the manufacturer's revolutions-per-mile figure when you need high accuracy.