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Tyre dimensions and speed change

Compare the geometric outside diameters of two tyre sizes. Estimate the speed change from the new-to-old diameter ratio.

Start with your own numbers

Editable example

An example is filled in. Replace it with your own data to calculate.

For 205/55 R16, enter tire width 205, aspect ratio 55 and rim size 16.

Original tyre width (mm)Enter original tyre width (mm), for example 205. Use the displayed unit; replace the example with your own value.

Original aspect ratio (%)Enter original aspect ratio (%), for example 55. Use the displayed unit; replace the example with your own value.

Original rim diameter (inches)Enter original rim diameter (inches), for example 16. Use the displayed unit; replace the example with your own value.

New tyre width (mm)Enter new tyre width (mm), for example 225. Use the displayed unit; replace the example with your own value.

New aspect ratio (%)Enter new aspect ratio (%), for example 45. Use the displayed unit; replace the example with your own value.

New rim diameter (inches)Enter new rim diameter (inches), for example 17. Use the displayed unit; replace the example with your own value.

Indicated speed (km/h)Enter indicated speed (km/h), for example 100. Use the displayed unit; replace the example with your own value.

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Example result

Change in outside diameter

0.379807%

Compares geometric size and estimated speed. It does not confirm fitment or completed speedometer calibration.

Original outside diameter631.9 mm

New outside diameter634.3 mm

Geometric circumference of the new tire1,992.71 mm

Estimated actual speed after changing tires100.38 km/h

Geometric outside diameter = rim inches × 25.4 + 2 × tire width × aspect ratio ÷ 100. Speed conversion uses new diameter ÷ old diameter. Load deformation, pressure, wear and original speedometer error are excluded. This does not establish legal modification or compatibility with brakes, wheel arches, load ratings or speed ratings.

Method and scope

Outside diameter equals rim inches times 25.4 plus twice tyre width times aspect ratio divided by 100. Estimated speed is indicated speed times new diameter divided by old diameter.

The aspect number is sidewall height as a percentage of tyre width, not a millimetre height. A larger rolling diameter increases distance per revolution.

Load deformation, pressure, wear and existing speedometer error are excluded. This does not confirm lawful modification, brake or wheel-arch clearance, load capacity or speed rating.

References:Michelin: tire size markings

What this helps you do

Compare the geometric outside diameters of two tyre sizes. Estimate the speed change from the new-to-old diameter ratio.

What to enter

Start with original tyre width (mm), then fill the other relevant fields in the units shown. The field hints explain the required format and examples.

How the calculation works

Outside diameter equals rim inches times 25.4 plus twice tyre width times aspect ratio divided by 100. Estimated speed is indicated speed times new diameter divided by old diameter.

Worked example

A 205/55 R16 tyre has a theoretical diameter of 631.9 mm. A 225/45 R17 tyre is 634.3 mm, giving about 100.38 km/h at an indicated 100 km/h under the ratio assumption.

Understanding the answer

The aspect number is sidewall height as a percentage of tyre width, not a millimetre height. A larger rolling diameter increases distance per revolution.

Scope and limitations