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Tyre Size Converter Calculator

Work out tyre size converter instantly with clear inputs, formula shown and shareable results.

About Tyre Size Converter Calculator

A 205/55 R16 tyre has a 205 mm section width, a sidewall 55% of that at 112.75 mm, and a 16 inch rim - giving an overall diameter of 631 mm. Rolling circumference of 1982 mm means 504 revolutions per kilometre, which is what the speedometer is calibrated against, so changing tyre size shifts the reading.

Formulas

Sidewall height

height = section width x aspect ratio / 100

Overall diameter

diameter = rim(in) x 25.4 + 2 x sidewall height

Worked example

With the default values below, Tyre Size Converter Calculator returns Overall Tyre Diameter of 631.9. Change any input above and every figure here is recalculated in your browser.

Inputs used

Section widthminimum 50 mm
205 mm
Aspect ratioaccepts 10–100 %
55 %
Rim diameterminimum 5 in
16 in

Results

Overall Tyre Diameter
631.9 mm
Sidewall Height
112.8 mm
Rolling Circumference
1,985 mm
Revolutions Per Kilometre
503.7

Step by step

  1. 1Values used
  2. 2Sidewall height
  3. 3Overall diameter
  4. 4Overall Tyre Diameter= 631.9 mm
  5. 5Sidewall Height= 112.8 mm
  6. 6Rolling Circumference= 1,985 mm
  7. 7Revolutions Per Kilometre= 503.7

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does changing tyre size affect the speedometer?

Because the reading is derived from wheel revolutions. A larger rolling circumference covers more ground per turn, so the display reads low.

What is a plus-size fit?

A larger rim with a lower aspect ratio, keeping overall diameter within about 3% so gearing and the speedometer stay correct.

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