Plus-sizing fitment study
What did the TireFitLab engine find?
TireFitLab computed 381 plus/minus candidates from 127 popular seed sizes using the same tolerance rule and geometry engine as the live calculator. 127 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% | 0.03% | 0.47% |
| +1" | 127 | 100% | 0.05% | 0.44% |
| +2" | 127 | 100% | 0.04% | 0.25% |
Closest engine matches
| Current size | Candidate | Step | Diameter delta | Speedometer error |
|---|---|---|---|---|
| 285/45 R22 | 235/60 R21 | -1" | +0.01% | +0.01% |
| 275/45 R22 | 195/70 R21 | -1" | +0.01% | +0.01% |
| 265/70 R17 | 305/65 R16 | -1" | +0.01% | +0.01% |
| 305/40 R22 | 245/55 R21 | -1" | +0.01% | +0.01% |
| 285/60 R18 | 245/75 R17 | -1" | +0.01% | +0.01% |
| 285/40 R22 | 195/65 R21 | -1" | +0.01% | +0.01% |
| 275/45 R21 | 195/70 R20 | -1" | +0.01% | +0.01% |
| 275/45 R21 | 185/60 R22 | +1" | -0.01% | -0.01% |
| 265/70 R16 | 305/65 R15 | -1" | +0.01% | +0.01% |
| 265/60 R18 | 195/75 R19 | +1" | -0.01% | -0.01% |
| 255/65 R17 | 255/60 R18 | +1" | -0.01% | -0.01% |
| 255/65 R17 | 255/70 R16 | -1" | +0.01% | +0.01% |
| 255/50 R20 | 255/55 R19 | -1" | +0.01% | +0.01% |
| 255/50 R20 | 255/45 R21 | +1" | -0.01% | -0.01% |
Method
The study uses the public TireFitLab seed 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
Planning a long summer drive?
Use the budget and running-cost tools before a trip, especially if the current tires are worn or the replacement size changes diameter.