Tire Size Numbers
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Definition
Tire size is encoded in the format Width/AspectRatio R Diameter (e.g., 205/55 R16), where width is in millimeters, aspect ratio is the sidewall height as a percentage of width, and diameter is the wheel rim size in inches.
Explanation
A tire marked 205/55 R16 has a tread width of 205 mm, a sidewall height of 113 mm (55% of 205), fits a 16-inch rim, and has a total outer diameter of approximately 632 mm. The mixed-unit format — metric width and percentage, but imperial rim diameter — reflects the tire industry's historical evolution across US and European standards. Load index and speed rating letters are often appended (e.g., 91V), indicating maximum load and speed.
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