🛞 Tyre Size Calculator
Compare your original tyre size with other sizes: diameter, real speed at the speedometer, ground clearance, and a list of equivalent sizes.
🔎 Decode a marking
Reference tables
Load index (kg per tyre)
| Index | Load | Index | Load |
|---|
Speed symbol
| Symbol | Max speed |
|---|
⚠️ Indicative calculation. The size fitted must be approved for your vehicle (certificate of conformity, registration document, manufacturer approval); if in doubt, ask a tyre professional. Non-approved sizes may fail inspection and cause insurance issues.
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About the Tyre Size Calculator
Changing tyre size (bigger rims, narrower winter tyres…) changes the wheel diameter, and therefore the displayed speed, the ground clearance and sometimes the car’s handling. This tool works out those differences from the tyre markings, checks the chosen tolerance and suggests equivalent sizes.
How to Use the Tool
- Enter the original size from the sidewall or registration document, e.g. 205/55 R16 91V.
- Add one or more sizes to compare, or click “Compare” in the list of suggested sizes.
- Pick the criterion: the French technical inspection one (the strictest) or a percentage of the diameter.
- Filter the suggestions by rim diameter or width, and decode a full marking in the last box.
Formulas and Rules Used
- Sidewall height: width × aspect ratio ÷ 100 (205/55 → 112.75 mm).
- Diameter: rim × 25.4 + 2 × sidewall; circumference = π × diameter; revolutions per km = 1,000,000 ÷ circumference.
- Speedometer: real speed = displayed speed × new diameter ÷ original diameter.
- French inspection: the TNPF equivalence table used by inspectors accepts a diameter within ±3% of the section height (per ETRTO), a width of 0 to +50 mm, a rim of 0 to +3 inches (winter: −20 to +50 mm and −1 to +3 inches) and a load index at least equal.
- Rule of thumb: many garages and websites accept ±3% of the diameter; it is a habit, not an official rule.
Tips
- Check the approval: allowed sizes appear on the certificate of conformity or come from the manufacturer.
- Keep load and speed indexes at least equal to the original ones (except winter tyres, within the allowed tolerance).
- Prefer a slightly smaller diameter to a larger one: the speedometer keeps showing a speed above the real one.
- Fit the same size on an axle, and ideally on all four wheels.
Common Mistakes and Limits
- Forgetting clearance: a wider tyre can rub in the wheel arch or on the suspension.
- Ignoring the load index when moving to a bigger rim: choose an XL version if needed.
- Theoretical sizes: the real diameter varies slightly with the tyre model, wear and pressure.
- No legal advice: the tool replaces neither the vehicle approval nor a professional’s advice.
Privacy & Security
- 100% in your browser: no data is sent to a server.
- No sign-up: free and instant.
- Works offline once the page has loaded.