Tire Size Calculator
Convert a tire spec like 225/45R17 into overall diameter, sidewall height, circumference and revolutions per mile.
How it works
Sidewall (in) = width × aspect ÷ 100 ÷ 25.4 · Diameter = wheel + 2 × sidewall
A tire code like 225/45R17 packs three numbers: section width in millimeters, aspect ratio (sidewall height as a percent of width), and wheel diameter in inches. Sidewall height converts to inches by dividing millimeters by 25.4; the overall diameter is the wheel plus two sidewalls, top and bottom. Circumference is π times the diameter, and dividing 63,360 inches per mile by the circumference gives revolutions per mile.
Worked example
For 225/45R17: sidewall = 225 × 0.45 = 101.25 mm ≈ 3.99 in. Overall diameter = 17 + 2 × 3.99 ≈ 24.97 in, circumference ≈ 78.45 in, and about 808 revolutions per mile.
Frequently asked questions
Why does tire diameter matter?
It sets your true road speed and odometer accuracy. Fitting a taller tire than stock makes the speedometer read low, while a shorter tire makes it read high, because the calibration assumes the original revolutions per mile.
What is the aspect ratio?
It is the sidewall height as a percentage of the tread width. A 45 means the sidewall is 45% of the 225 mm width; lower numbers mean shorter, sportier sidewalls and higher numbers mean taller, cushier ones.
How do I keep my speedometer accurate?
Choose a replacement tire with an overall diameter within about 3% of the original. Larger deviations noticeably shift speedometer and odometer readings and can affect some driver-assist systems.
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This calculator is for educational purposes. Double-check important results before acting on them.