| Diameter | Circumference |
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What Tire Circumference Tells You
Circumference is the distance a tire covers in a single turn, found by multiplying the overall diameter by pi. It is the bridge between tire size and everything that depends on how far each rotation carries you: revolutions per mile, odometer and speedometer readings, and the gearing that sets engine RPM at speed.
From Rotations to Distance
Multiply circumference by the number of revolutions and you get distance traveled, which is exactly how a vehicle measures speed and mileage from wheel-speed sensors. Change the tire size and you change the circumference, so the car covers more or less ground per rotation than it thinks, throwing the speedometer and odometer off until recalibrated.
Loaded vs Unloaded
The printed size gives an unloaded diameter, but under the weight of the car the tire flattens slightly at the contact patch, shrinking the effective rolling circumference a little. For precise gearing or speedometer work, the loaded rolling figure is more accurate, though the nominal value is close enough for most planning.
Frequently Asked Questions
How do I get my tire diameter?
Decode it from the tire size with a tire diameter calculator, or measure ground-to-axle height and double it for the loaded rolling diameter.
Why does circumference affect my speedometer?
The car counts wheel turns and assumes a fixed circumference. A larger tire travels farther per turn, so the speedometer reads low until recalibrated.
What is the link to revolutions per mile?
Revolutions per mile equal the inches in a mile, 63,360, divided by the circumference in inches. Bigger tires turn fewer times per mile.