How TireFitLab calculates fitment
Every figure is computed from the tire size in canonical millimetres — the same engine powers the calculator, the size pages and the comparisons, so the numbers are identical everywhere.
Formulas
| Sidewall height | width × aspect ÷ 100 |
|---|---|
| Overall diameter | rim + 2 × sidewall |
| Circumference | π × diameter |
| Speedometer error | (newØ − oldØ) ÷ oldØ × 100% |
Tolerance rules
The following tolerances are a working baseline and must be verified per market against a primary source.
| Market | +max | −max | Source |
|---|---|---|---|
| EU general (±3%) | +3% | −3% | ETRTO / common EU practice (verify per market) |
| Poland | +1.5% | −2% | PL national guidance (verify) |
| US (~3%) | +3% | −3% | Tire industry common practice / T&RA (verify) |
| AWD / 4WD (tight) | +2% | −2% | Drivetrain rolling-radius guidance (verify) |
Last reviewed: 2026-06-21
Note: geometric estimate. Always verify physical clearance before fitting.
What changed
- Reviewed deterministic geometry, load/speed references, sitemap inclusion and localized page shell.