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About this tool
Convert 205/55R16 style metric tyre sizes to 31x10.50R15 inch flotation sizes and back, with speedometer error.
The two tyre sizing systems describe the same rubber in different terms, and this converter moves between them exactly. A metric size such as 205/55R16 gives section width in millimetres, sidewall height as a percentage of that width, and rim diameter in inches, so overall diameter is rim × 25.4 + 2 × (width × aspect ÷ 100). An inch or flotation size such as 31x10.50R15 states overall diameter and width directly in inches, from which aspect ratio falls out as (diameter − rim) ÷ 2 ÷ width. Enter either and you get the other, plus rolling circumference, revolutions per kilometre and the speedometer error against your factory size.
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Uses the exact 25.4 mm inch, so a converted size matches the manufacturer's published dimensions rather than a rounded approximation.
Shows the true speed at any indicated reading and how many kilometres the odometer displays over 100 real ones.
Flags a change that moves overall diameter beyond about 3% of stock, where ABS and stability-control calibration start to drift.
Sidewall height is 205 × 0.55 = 112.75 mm, and the 16-inch rim is 406.4 mm, so overall diameter is 406.4 + 2 × 112.75 = 631.9 mm, or 24.9 inches. Section width is 205 ÷ 25.4 = 8.07 inches, making the flotation equivalent roughly 24.9x8.07R16.
31 inches overall diameter, 10.50 inches section width, on a 15-inch rim. The sidewall is (31 − 15) ÷ 2 = 8 inches, which against a 10.50-inch width is a 76% aspect ratio — so the nearest metric size is about 267/76R15, commonly sold as 265/75R15.
Keep the overall diameter within about 3% of the factory size. A speedometer is calibrated to the original tyre's rolling circumference, so a diameter 3% larger makes you travel 3% faster than the dial says — 103 km/h when it reads 100 — and makes the odometer under-record by the same proportion.
No, and that is the point of plus-sizing. Going from 205/55R16 to 225/45R17 adds 20 mm of width and an inch of rim but cuts the aspect ratio, so overall diameter changes by only about 2 mm — under half a percent. Width and diameter are independent as long as the aspect ratio moves to compensate.
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