Calculates tyre dimensions from the sidewall markings — e.g. 205/50/R15. Enter the three numbers from your tyre to get overall diameter, loaded radius, rolling circumference and speedo correction factor.
Tyre Markings
Enter the three numbers from your tyre sidewall. Example: 205 / 50 / R15
205
/
50
/
R15
Nominal section width at the widest point of the inflated unloaded tyre, sidewall to sidewall. Actual width varies with the rim width fitted
Sidewall height as a percentage of the section width
Wheel/rim diameter in inches
For metric slick sizing such as 200/580R15, where the middle number is overall diameter in millimetres rather than an aspect ratio. Leave blank for a normal sidewall marking
Roll the car one full wheel revolution at operating pressure, mark the contact patch at both ends, measure the distance. Average three runs. If entered, this overrides the calculated estimate
Static deflection under load. Default 13% is typical for a passenger or competition radial at normal load and pressure. Lower it for high pressures or light loads
Same override as above. Use it when the tyre you are comparing against is a metric slick
Results
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Overall diameter (mm)
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Overall diameter (in)
—
Free radius (mm) — unloaded
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Static deflection (mm) — estimated
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Static loaded radius (mm)
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Effective rolling radius (mm)
—
Rolling circumference (mm) — use this for gearing, speedo and logger calibration
—
Sidewall height (mm)
—
Sidewall height (in)
Three radii, and which one to use.
Free radius is half the overall diameter with no load on the tyre. Static loaded radius is hub centre to ground with the car sitting on it, so free radius minus deflection. Effective rolling radius is the one that governs distance travelled per revolution, and it sits between the two: the tread band does not shorten when the contact patch flattens, so the tyre covers more ground than the static loaded radius implies. Calculated here as free radius minus two thirds of the deflection, which lands at roughly 0.97 of free radius for a typical fitment.
Use rolling circumference for gearing, speedo correction and logger wheel speed calibration. Using free circumference overstates speed by roughly 1 to 2%. Using static loaded radius understates it by roughly 2 to 3%.
Free radius is half the overall diameter with no load on the tyre. Static loaded radius is hub centre to ground with the car sitting on it, so free radius minus deflection. Effective rolling radius is the one that governs distance travelled per revolution, and it sits between the two: the tread band does not shorten when the contact patch flattens, so the tyre covers more ground than the static loaded radius implies. Calculated here as free radius minus two thirds of the deflection, which lands at roughly 0.97 of free radius for a typical fitment.
Use rolling circumference for gearing, speedo correction and logger wheel speed calibration. Using free circumference overstates speed by roughly 1 to 2%. Using static loaded radius understates it by roughly 2 to 3%.
Accuracy of the estimate: about ±2%.
The deflection figure is a proportion of sidewall height, not a measurement. Real deflection depends on vertical load, inflation pressure and carcass construction, none of which are inputs here. Tyre growth from centrifugal loading at high speed is not modelled and increases effective radius further, so a calculated figure will read low at the top end of a long straight.
If the number matters, measure it. Enter a measured rolling circumference above and it overrides everything on this page. The measurement method and why a wrong circumference constant corrupts every wheel speed reading in a session are covered on the Data Channel Reference.
Slicks: the 13% default is not validated. It was set for a passenger or competition radial. A slick carcass at competition pressure almost certainly deflects less, and on a 200/580R15 the gap between 13% and 6% is 1.6% of rolling circumference, which is most of the error band on its own. Until you have measured one, treat a calculated slick figure as indicative rather than a number to gear a car around. Use the deflection override once you know what yours actually does.
If the number matters, measure it. Enter a measured rolling circumference above and it overrides everything on this page. The measurement method and why a wrong circumference constant corrupts every wheel speed reading in a session are covered on the Data Channel Reference.
Slicks: the 13% default is not validated. It was set for a passenger or competition radial. A slick carcass at competition pressure almost certainly deflects less, and on a 200/580R15 the gap between 13% and 6% is 1.6% of rolling circumference, which is most of the error band on its own. Until you have measured one, treat a calculated slick figure as indicative rather than a number to gear a car around. Use the deflection override once you know what yours actually does.
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