Compare two tire sizes and see how a size change affects overall diameter and your speedometer reading.
Tire Size Calculator
Automotive
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Find your current tire specs on the sidewall (e.g., 225/45R17).
Enter the width (mm), aspect ratio (%), and rim (in) for your current tire.
Enter the same three values for the new tire you are considering.
Click Calculate to compare diameters and speedometer impact.
Tire size 225/45R17 means 225mm wide, 45% aspect (sidewall height = width × aspect), on a 17-inch rim. Overall diameter = (width ÷ 25.4) × aspect × 2 + rim (in inches). Switching sizes changes the rolling circumference; a larger tire makes your actual speed higher than the speedometer shows.
Most experts recommend staying within ±3% of the original diameter to avoid affecting gearing, ABS, and traction control. Smaller is safer.