Plus-sizing fitment study
What did the TireFitLab engine find?
TireFitLab computed 381 plus/minus candidates from 127 popular sizes using the same tolerance rule and geometry engine as the live calculator. 68 base sizes have at least one candidate within 0.5% overall diameter, but every result still needs load-rating, rim-width, and physical-clearance checks.
Coverage by rim step
| Step | In-tolerance candidates | Seed coverage | Median diameter delta | Largest accepted delta |
|---|---|---|---|---|
| -1" | 127 | 100% | 0.65% | 1.48% |
| +1" | 127 | 100% | 0.6% | 1.47% |
| +2" | 127 | 100% | 1.01% | 2.98% |
Closest engine matches
| Current size | Candidate | Step | Diameter delta | Speedometer error |
|---|---|---|---|---|
| 255/65 R17 | 255/60 R18 | +1" | -0.01% | -0.01% |
| 265/60 R18 | 245/70 R17 | -1" | -0.05% | -0.05% |
| 255/55 R18 | 235/65 R17 | -1" | -0.05% | -0.05% |
| 245/65 R18 | 245/60 R19 | +1" | +0.12% | +0.12% |
| 245/65 R18 | 245/70 R17 | -1" | -0.12% | -0.12% |
| 245/70 R17 | 245/75 R16 | -1" | -0.12% | -0.12% |
| 245/70 R17 | 245/65 R18 | +1" | +0.12% | +0.12% |
| 245/60 R18 | 245/55 R19 | +1" | +0.12% | +0.12% |
| 245/60 R18 | 245/65 R17 | -1" | -0.12% | -0.12% |
| 245/65 R17 | 245/70 R16 | -1" | -0.12% | -0.12% |
| 245/65 R17 | 245/60 R18 | +1" | +0.12% | +0.12% |
| 245/70 R16 | 245/75 R15 | -1" | -0.12% | -0.12% |
| 245/70 R16 | 245/65 R17 | +1" | +0.12% | +0.12% |
| 245/45 R20 | 245/40 R21 | +1" | +0.12% | +0.12% |
Method
The study uses the public TireFitLab size corpus and calls plusMinusOptions() for each base size with −1, +1, and +2 inch rim steps. Geometry is computed in canonical millimetres, then aggregated by rim step. No vehicle-fitment database or scraped manufacturer data is used.
A candidate can match diameter closely and still be wrong for a specific car. Confirm rim width, load index, speed rating, suspension and arch clearance, brake clearance, and local road rules before fitting.
More tools
- Tire size calculator
- Plus sizing guide
- Rim width and tire fitment guide
- Tire & wheel reference guides
Seasonal check
Winter tire rules coming up?
Check tread depth, markings and local winter requirements before the first cold spell instead of waiting for the appointment rush.