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Decode metric tire sizes (225/55R17) and calculate overall diameter, sidewall height, and revolutions per mile.
Tire size is expressed in a standardized format that encodes width, aspect ratio, construction type, and rim diameter. The most common format is metric (e.g., 225/55R17): 225mm section width, 55% aspect ratio, radial construction, 17-inch rim.
| Size | OD (in) | Common Vehicle |
|---|---|---|
| 195/65R15 | 25.0" | Compact sedans (Honda Civic, Toyota Corolla) |
| 205/55R16 | 24.9" | Mid-size sedans (VW Jetta, BMW 3 Series) |
| 225/55R17 | 26.7" | Crossovers, SUVs (Honda CR-V, Toyota RAV4) |
| 245/45R18 | 26.7" | Performance sedans (Audi A4, BMW 5 Series) |
| 265/70R17 | 31.6" | Pickup trucks, SUVs (Ford F-150, Jeep Cherokee) |
| 275/55R20 | 31.9" | Full-size trucks (GMC Sierra, Ram 1500) |
Formula
Overall Diameter (mm) = (Rim Diameter × 25.4) + 2 × (Width × Aspect Ratio ÷ 100)Rim Diameter = rim diameter in inches, multiplied by 25.4 to convert to mm
Width = tire section width in millimeters (the first number in the tire size)
Aspect Ratio = sidewall height as a percentage of section width (the second number)
Worked Example
225/55R17 — overall diameter calculation
Did you know? Changing to a tire with an overall diameter more than 3% different from the stock size can cause speedometer inaccuracy and may interfere with ABS and traction control systems (source: Tire Rack).
Sources
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