Look up the overall diameter, section width and rim size in inches for popular metric tire sizes (e.g. 225/45R17).
Automotive • metric ↔ inches
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The metric "P-metric" tire code (e.g. 225/45R17) was standardised in the late 1970s and is now used worldwide, but the older US flotation system describes tires directly in inches (e.g. 31×10.50R15). Drivers swapping wheels, fitting larger tires, or comparing options often need the overall diameter in inches — which also affects speedometer accuracy and gearing.
Overall diameter is calculated as: rim diameter + 2 × (section width × aspect ratio). For 225/45R17 that is 17 + 2 × (225 × 0.45 ÷ 25.4) ≈ 24.97 inches. This table lists popular sizes with their computed diameter, section width and rim so you can match a metric size to its real-world inch dimensions at a glance.
A tyre code such as 225/45R17 mixes three unit systems in one string, which is why it resists intuition. The first number is section width in millimetres. The second is the aspect ratio — sidewall height as a percentage of that width, not a measurement in its own right. The R denotes radial construction, and the final number is wheel diameter in inches. Because sidewall height is derived from width, changing the width alters the sidewall too, which is what makes plus-sizing calculations non-obvious. Overall diameter must stay within roughly 3% of original, or the speedometer and ABS calibration drift.
Sidewall height (mm) = width × (aspect ratio ÷ 100)Overall diameter (mm) = (wheel inches × 25.4) + (2 × sidewall height)Revolutions per mile = 1,609,344 ÷ (π × diameter in mm)where:
Assumptions: Keep overall diameter within about 3% of the original or the speedometer reads incorrectly and traction-control calibration is affected. Also confirm load index and speed rating, which the size code does not include.
Break the code apart, compute overall diameter, and test a plus-size alternative.
Result225/45R17 = 634.3 mm (24.97 in) overall diameter
That 3.55% increase exceeds the 3% guideline, so the speedometer would under-read by about 2 mph at 60. Fitting it would also raise the car slightly and soften the ride — a real trade-off, but one to make deliberately rather than by accident.