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Tyre Size Comparison Calculator

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

About Tyre Size Comparison Calculator

Overall diameter is the rim diameter plus twice the sidewall height, and sidewall height is width times the aspect ratio. Changing width therefore changes diameter unless the aspect ratio is adjusted — the basis of plus-sizing calculations.

Formula

Tyre diameter

diameter = rim (mm) + 2 x width x aspect ratio / 100

Worked example

With the default values below, Tyre Size Comparison Calculator returns Overall diameter change of 3.482%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Current tyre width
205 mm
Current aspect ratio
55 %
Current rim diameter
16 in
New tyre width
225 mm

Results

Overall diameter change
3.482% %
Current overall diameter
631.90 mm
New overall diameter
653.90 mm
Aspect ratio to keep the diameter
50.1% %

Step by step

  1. 1Values used
  2. 2Tyre diameter
  3. 3Overall diameter change= 3.482 %
  4. 4Current overall diameter= 631.90 mm
  5. 5New overall diameter= 653.90 mm
  6. 6Aspect ratio to keep the diameter= 50.1 %

Assumptions and limits

  • The result is derived only from the 4 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 3 decimal places, 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

How much diameter change is acceptable?

Within 2-3% of original. Beyond that the speedometer reads wrong, ABS and stability calibration drift, and gearing changes noticeably.

What is plus-sizing?

Fitting a larger rim with a lower aspect ratio to keep the overall diameter constant. It improves steering response at the cost of ride comfort.

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