For 225/45 R17, sidewall height is 225 × 0.45 = 101.25 mm. Add two sidewalls to a 17-inch rim: 101.25 × 2 + 431.8 = 634.3 mm. For 235/40 R18, sidewall height is 94.0 mm and rim diameter is 457.2 mm, producing 645.2 mm. Divide 645.2 by 634.3 to get 1.0172, or a 1.72% diameter increase.
Worked example with numbers
The tyre size comparison tool performs the diameter ratio and gives an indicated-versus-actual guide. Enter the full three-part marking, not just rim size. Width is millimetres, aspect ratio is a percentage of width, and rim diameter is inches; a 45 sidewall is not 45 mm on a 225/45 tyre.
How to make the calculation
The odometer shares the same wheel-rotation assumption. If the replacement circumference is larger, each wheel turn travels farther than the system expects, so the odometer can record less distance than actually travelled. If circumference is smaller, the indicated distance can rise faster. The effect is approximate because real rolling radius changes under load, pressure, speed, and tyre construction.
Check the units and assumptions
Do not infer a safe fit from diameter alone. A tyre can have a similar diameter but a different width that rubs suspension or bodywork. Do not ignore load index, speed rating, wheel width, brake clearance, spare-wheel limitations, or local rules. A speedometer may already have a manufacturer tolerance, so the calculated percentage is not a calibrated road-speed test.
Common mistakes
The geometry is suitable for comparing nominal tyre sizes. It cannot certify wheel fitment, alignment, ABS operation, warranty compliance, or legal roadworthiness. Check the vehicle placard, owner documentation, and a qualified tyre professional for those questions; calculations explain the size change but do not replace an approval.
When this result does not apply
The European Tyre and Rim Technical Organisation publishes technical standards for tyre and rim applications; its standards information identifies why nominal markings alone are not a complete fitment decision. The formula here is intentionally limited to the labelled dimensions.
Source for the external fact
Use the tyre-diameter calculator for one marking and the length converter when checking millimetres against inches.
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