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Tyre Size Metric to Inch Converter

Convert 205/55R16 style metric tyre sizes to 31x10.50R15 inch flotation sizes and back, with speedometer error.

About this tool

Tyre Size Metric to Inch Converter

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.

How to use Tyre Size Metric to Inch Converter

  1. 1

    Open Tyre Size Metric to Inch Converter on AltFTool — it loads instantly in your browser.

  2. 2

    Type a sidewall size such as 205/55R16 or 31x10.50R15 into Paste a size from the sidewall and press Read size.

  3. 3

    Tick Compare against the factory size and fill the stock section width, aspect ratio and rim diameter.

  4. 4

    Read the overall diameter, rolling circumference and revolutions per km, plus the 3% fitment window check and true speed at the indicated reading.

Why use Tyre Size Metric to Inch Converter

  • Both directions, exactly

    Uses the exact 25.4 mm inch, so a converted size matches the manufacturer's published dimensions rather than a rounded approximation.

  • Speedometer and odometer error

    Shows the true speed at any indicated reading and how many kilometres the odometer displays over 100 real ones.

  • Fitment window checked

    Flags a change that moves overall diameter beyond about 3% of stock, where ABS and stability-control calibration start to drift.

Common use cases

  • Your SUV came with 265/70R17 and the off-road tyre you want is listed only as 31x10.50R15 — you need them in the same language to compare.
  • You are moving from 205/55R16 to 225/45R17 and want to confirm the overall diameter stays within the fitment window.
  • Your speedometer has read optimistic since a tyre change and you want to know the true speed when the dial shows 100.

Frequently asked questions

How do I convert 205/55R16 to inches?

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.

What does 31x10.50R15 mean?

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.

How much can I change tyre size without affecting the speedometer?

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.

Does a wider tyre always have a bigger overall diameter?

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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