Tire Size & Speedometer Error Calculator
Decode a tire size into its overall diameter, circumference, and revolutions per mile — or compare two sizes to see how much a tire swap throws off your speedometer and odometer.
Enter Your Tire Size
Result
Enter a tire size to see its dimensions
How This Calculator Works
A tire code like 225/45R17 packs in three numbers: width in millimeters, aspect ratio (sidewall height as a % of width), and rim diameter in inches. The calculator converts everything to a common unit to find the tire's overall diameter, then derives circumference and how many times it revolves per mile or kilometer.
Your speedometer converts wheel revolutions to speed using a calibration based on the original tire's diameter. Fit a different-diameter tire, and the same number of revolutions now corresponds to a different actual speed — the error is simply the percentage change in overall diameter.
Frequently Asked Questions
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