Tire Size Calculator
The Tire Size Calculator compares two tire sizes (in the standard width/aspect-ratio/rim format) to show the difference in overall diameter and how it affects speedometer accuracy.
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How This Tool Works
Enter both tire sizes in width/aspect-ratio/rim format (found on your tire's sidewall, e.g. 225/60R16) to compare their overall diameters.
Formula & Method
Sidewall height (mm) = tire width × (aspect ratio ÷ 100). Overall diameter (in) = (sidewall height × 2 ÷ 25.4) + rim diameter.
Example Calculation
A 225/60R16 tire (25.3 in diameter) compared to a 235/55R17 (25.98 in diameter) is about 2.7% larger — your speedometer would read 60 mph at a true 61.6 mph.
Frequently Asked Questions
What do the numbers in a tire size mean?+
In "225/60R16," 225 is the tire width in millimeters, 60 is the aspect ratio (sidewall height as a % of width), and 16 is the rim diameter in inches.
How much diameter difference is safe?+
Most tire professionals recommend staying within about 3% of the original overall diameter to avoid speedometer errors and clearance or handling issues.
Does a different tire size affect my speedometer?+
Yes — since speedometers calculate speed from wheel rotations, a larger tire diameter travels farther per rotation, making your speedometer read slower than your true speed.
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