Tire Size Calculator
Tire specs like 225/45R17 are width / aspect ratio / rim diameter. Compare two specs to see how diameter, ground clearance, and your speedometer change.
Comparison
Diameter difference
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Original diameter
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New diameter
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Original sidewall
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New sidewall
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Speedometer reads 60, actual
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Revs/mile change
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How to read a tire spec
225/45R17 means: 225 mm wide, sidewall is 45% of the width (about 101 mm), and the rim diameter is 17 inches. Multiply sidewall × 2 + rim → total tire diameter.
Why diameter matters
Your speedometer reads off the wheel rotation, calibrated for the original tire size. A larger total diameter means each rotation covers more ground — so your actual speed is higher than the speedo shows. The same applies to the odometer (under-counts miles) and any traction-control systems calibrated to your wheel speed.
Convention: stay within ±3% of the OEM diameter for safe substitution. Beyond ±3% you should expect noticeable speedometer error and potential clearance issues.
Plus-sizing
"Plus 1" means going up one rim size while keeping the same overall diameter — typically a wider, lower-profile tire. Example: 225/45R17 → 235/40R18 keeps diameter constant while improving handling.