Decode tyre size codes and calculate diameter, sidewall height, circumference and revolutions per kilometre — with both metric and imperial outputs.
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Enter a full tyre size code (e.g. 225/55R17) — or manually enter width, aspect ratio and rim diameter for custom sizes.
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For 225/55R17: 225 = section width in mm, 55 = aspect ratio (sidewall height as % of width), R = radial construction, 17 = rim diameter in inches. Sidewall height = 225 × 55% = 123.75 mm. Overall diameter = (17 × 25.4) + (2 × 123.75) = 431.8 + 247.5 = 679.3 mm.
The aspect ratio is the sidewall height expressed as a percentage of the tyre width. A 225/55 tyre: sidewall = 55% × 225 mm = 123.75 mm. Lower ratios (30, 35, 40) mean shorter, stiffer sidewalls with sportier handling. Higher ratios (65, 70) give more cushioning on rough surfaces. Most standard car tyres use 45–65.
Your speedometer, odometer and ABS calibrate to the original tyre's rolling circumference. A larger tyre travels further per revolution, making your speedometer read low and odometer record fewer miles. Most manufacturers allow ±2–3% change in overall diameter before readings become significantly inaccurate or safety systems are affected.
Plus sizing uses a larger rim diameter (e.g. from 15" to 17") with a lower-profile tyre (lower aspect ratio) to maintain the same overall tyre diameter. A "+2" upgrade keeps rolling circumference identical for accurate speedometer readings while gaining handling precision and visual appearance improvement.
These appear after the size code — e.g. 205/55R16 91H. The load index (91) specifies the maximum weight each tyre supports (91 = 615 kg per tyre). The speed rating letter (H) is the maximum sustained speed (H = 210 km/h, V = 240, W = 270, Y = 300 km/h). Always match or exceed your vehicle's original ratings.
