Tyre Size & Speedometer Error
Calculate tyre diameter, sidewall and circumference from the size code and see the speedometer error when you change tyre size, free online.
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A tyre marked 205/55 R16 has a width of 205 mm, a sidewall height of 55% of the width and fits a 16-inch rim. Overall diameter = rim (in × 25.4) + 2 × sidewall. A larger diameter makes the speedometer read low (you go faster than shown). Keep the change within ±3%.