Tyre Size Calculator 2026: Diameter, Compare & Speedo Error
205/55 R16 stands 631.9 mm tall. Enter any size for sidewall, diameter and rolling circumference, then add a second size to see the speedometer difference.
Enter Your Tyre Size
Dimensional Breakdown
Tyre Cross Section
Compare With Another Size: Speedometer Difference
| Property | Original | New | Difference |
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Compatible Alternative Sizes
Sizes whose rolling circumference is within 3% of your original. Green = closer match. Speedo error is computed from rolling circumference, not geometric.
| Tyre Size | Rolling Circ (mm) | Difference | Speedo Error |
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Calculations use standard tyre geometry plus an estimated loaded-deflection model for rolling circumference. Actual dimensions vary by manufacturer, load, pressure, wear and speed. Always verify with your vehicle manual and tyre dealer.
How we calculate rolling circumference
Most tyre calculators quote the geometric circumference — simply π multiplied by the unloaded overall diameter. That number is easy to compute and matches the figure on a manufacturer spec sheet, but it is not the distance the tyre actually travels in one turn. A tyre carrying a car flattens at the contact patch, so its effective rolling circumference is smaller than the geometric one.
A common shortcut is to apply a flat 96–97% correction to the geometric figure. That is better than ignoring the effect, but it is still wrong, because the amount a tyre squashes depends on how much sidewall it has. A tall 60-series sidewall deflects far more than a low 30-series one, so two tyres with an almost identical overall diameter can have rolling circumferences over 1% apart — enough to matter for your speedometer, odometer, ABS/ESP calibration and final-drive feel.
Our model estimates the single-sided sidewall deflection f and computes rolling circumference as RC = π × (unloaded diameter − f). The deflection scales with the section height, the share of the tyre's rated load it is carrying, and inflation pressure: a heavier load deflects more, a higher pressure deflects less. The baseline deflection fraction is adapted from published loaded-radius practice for pneumatic tyres and calibrated to passenger radials, with the defaults set to a typical road load (80% of rated) and 2.4 bar. You can change both in the Advanced panel.
Limitations, stated plainly: true rolling circumference is not a single fixed number. It rises slightly with speed as the tyre grows under centrifugal force, shifts with temperature and wear, and varies between tyre constructions and brands. The value here is a calibrated estimate, not a certified measurement — use it to compare sizes and sanity-check speedometer impact, then confirm anything safety- or legality-critical with the tyre maker's data.
Pick width, profile and rim and the calculator gives the sidewall height, overall diameter, circumference and revolutions per kilometre straight away. For 205/55 R16 the sidewall is 205 x 0.55 = 112.75 mm and the diameter is 16 x 25.4 + 2 x 112.75 = 631.9 mm. The comparison panel underneath is already open: choose the size you are thinking of fitting and it shows the speedometer difference and what an indicated 100 km/h really is. It works from rolling circumference, the distance a loaded tyre actually covers per turn, because that is what the speedometer and wheel-speed sensors count.
Tyre Size Calculator
Work out the dimensions of any metric tyre size and compare two sizes side by side. Understand what each number in a tyre size marking means and find equivalent sizes for your vehicle.
What This Tool Does
Enter any tyre size (e.g., 205/55 R16) and the calculator shows the full dimensional breakdown: overall diameter, sidewall height, tread width, and circumference. Compare two tyre sizes side by side to see how a change affects speedometer accuracy, ground clearance, and ride comfort.
How to Use It
- Enter your current tyre size in the standard format (width/aspect ratio R rim diameter).
- Optionally enter a second tyre size to compare.
- View the dimensional breakdown and percentage differences.
- Check the speedometer correction — a different tyre diameter means your speedometer may read faster or slower than actual speed.
Worked comparison: 205/55 R16 to 225/45 R17
A common upsize. The diameter goes from 631.9 mm to 634.3 mm, only 0.4% on paper. On the estimated rolling circumference the gap is about 0.7%, because the lower 45-series sidewall squashes less under load, so an indicated 100 km/h becomes roughly 100.7 km/h. That is well inside the 3% guideline. Moving to 225/40 R18 instead gives an estimated rolling difference of about 1.4%.
Understanding Tyre Size Markings
A tyre marked 205/55 R16 91V means: 205 mm tread width, 55% aspect ratio (sidewall height is 55% of width), radial construction, 16-inch rim, 91 load index (615 kg max), and V speed rating (240 km/h max). Choosing the wrong size can affect handling, fuel economy, and legal compliance.
How We Calculate Rolling Circumference
Most tyre calculators quote the geometric circumference — π times the unloaded diameter — and then treat it as the distance the tyre actually covers per turn. It isn’t. A tyre carrying a car flattens slightly where it meets the road, so the radius it rolls on is smaller than the radius it measures. The distance per revolution is the rolling circumference, and that is the number your speedometer, odometer and wheel-speed sensors are built around.
Some tools improve on the raw geometry by knocking a flat 96–97% off it. That is closer, but still wrong, because it assumes every tyre squashes by the same proportion. It doesn’t: a tall 60- or 70-series sidewall deflects far more than a low 30-series one, so a single percentage can’t fit both. That is exactly why two tyres with an almost identical overall diameter can still spin at meaningfully different rates.
We start from the geometry and subtract a deflection term once (the tyre only flattens on the ground side):
Rolling circumference = π × (unloaded diameter − deflection)
The deflection is estimated as a fraction of the sidewall height, scaled by how hard the tyre is loaded and how much pressure it holds. The base fraction comes from long-standing deflection practice used for agricultural and load-rated tyres, calibrated here so it lands on the measured behaviour of ordinary passenger radials. Lower pressure or heavier load increases the squash; higher pressure reduces it. The result is held inside sane physical limits so unusual inputs can’t produce nonsense.
Treat the output as a good estimate, not a certified figure. Real rolling circumference also shifts with speed (the tyre grows at motorway pace), temperature, tread wear and how the specific tyre is built. For anything legal or safety-critical, confirm against the manufacturer’s data and your vehicle handbook.
Data Sources
Tyre dimension calculations follow ISO 4000-1 standards and ETRTO (European Tyre and Rim Technical Organisation) specifications. The rolling-circumference model and speedometer-error figures are computed as described above.