Fitment & Wheels
Tire Size & Speedometer Calibrator
Compare overall diameter, sidewall stretch, revs per mile, and speedometer error percentage across aftermarket tire profiles.
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How to Use This Tool
- Select your factory (stock) tire width, aspect ratio, and rim diameter.
- Enter your proposed aftermarket tire size in the second column.
- Review overall diameter, sidewall height, revs per mile, and ride height change.
- Use the speedometer table to see indicated vs. actual speed from 20–90 MPH.
The Formula & Physics Behind the Math
Sidewall height in millimeters equals section width × aspect ratio ÷ 100. Overall tire diameter equals rim diameter (inches) plus twice the sidewall height (converted to inches). Circumference = π × diameter. Revs per mile = 63,360 ÷ circumference (inches). Speedometer error scales linearly with diameter ratio: actual speed = indicated speed × (new diameter ÷ stock diameter).
Frequently Asked Questions
Why does a taller tire change my speedometer reading?
Your speedometer counts driveshaft or wheel rotations per mile. A taller tire travels farther per revolution, so the car moves faster than the gauge shows at the same RPM.
What is a safe diameter change for daily driving?
Most enthusiasts stay within ±3% of stock overall diameter to preserve speedometer accuracy, ABS calibration, and gearing. Larger changes may require a tune or gear correction.
Do plus-sizing wheels affect diameter?
Plus-sizing typically keeps overall diameter similar by lowering aspect ratio while increasing rim diameter. Always calculate both setups — rim size alone does not define rolling diameter.