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Plus-sizing fitment study

# 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](https://tirefitlab.com/285-45-r22/) | `235/60 R21` | -1" | +0.01% | +0.01% |
| [275/45 R22](https://tirefitlab.com/275-45-r22/) | `195/70 R21` | -1" | +0.01% | +0.01% |
| [265/70 R17](https://tirefitlab.com/265-70-r17/) | `305/65 R16` | -1" | +0.01% | +0.01% |
| [305/40 R22](https://tirefitlab.com/305-40-r22/) | `245/55 R21` | -1" | +0.01% | +0.01% |
| [285/60 R18](https://tirefitlab.com/285-60-r18/) | `245/75 R17` | -1" | +0.01% | +0.01% |
| [285/40 R22](https://tirefitlab.com/285-40-r22/) | `195/65 R21` | -1" | +0.01% | +0.01% |
| [275/45 R21](https://tirefitlab.com/275-45-r21/) | `195/70 R20` | -1" | +0.01% | +0.01% |
| [275/45 R21](https://tirefitlab.com/275-45-r21/) | `185/60 R22` | +1" | -0.01% | -0.01% |
| [265/70 R16](https://tirefitlab.com/265-70-r16/) | `305/65 R15` | -1" | +0.01% | +0.01% |
| [265/60 R18](https://tirefitlab.com/265-60-r18/) | `195/75 R19` | +1" | -0.01% | -0.01% |
| [255/65 R17](https://tirefitlab.com/255-65-r17/) | `255/60 R18` | +1" | -0.01% | -0.01% |
| [255/65 R17](https://tirefitlab.com/255-65-r17/) | `255/70 R16` | -1" | +0.01% | +0.01% |
| [255/50 R20](https://tirefitlab.com/255-50-r20/) | `255/55 R19` | -1" | +0.01% | +0.01% |
| [255/50 R20](https://tirefitlab.com/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](https://tirefitlab.com/tire-size-calculator/)
- [Plus sizing guide](https://tirefitlab.com/learn/plus-sizing/)
- [Rim width and tire fitment guide](https://tirefitlab.com/learn/rim-width-tyre-fitment/)
- [Tire & wheel reference guides](https://tirefitlab.com/learn/)

[Last reviewed: 2026-06-23](https://tirefitlab.com/methodology/)

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.

[Estimate tire budget](https://tirefitlab.com/tire-budget-calculator/)

## More in this topic: Size validation

Decode markings, compare sizes, and understand diameter, load and speed changes.

- [**How to read a tire size** How to read a tire size: decode every number and letter in P205/55R16 91V — width, aspect ratio, rim diameter, load index, speed rating.](https://tirefitlab.com/learn/how-to-read-tire-size/)
- [**Tire type guide — P, LT, metric, radial & flotation** What do the letters in a tire size mean? P, LT, metric, radial (R) and flotation tire types explained.](https://tirefitlab.com/learn/tire-types/)
- [**Tire sidewall markings decoder** Complete guide to every marking on a tire sidewall: size string, construction code, load index, speed rating, DOT date, M+S, 3PMSF snowflake, UTQG, max pressure, country of manufacture, and brand-specific symbols.](https://tirefitlab.com/learn/tyre-sidewall-markings/)
- [**Tire load index chart** Use the tire load index chart to convert ratings 60–130 into kg and lb, then verify the vehicle placard and axle-load requirements.](https://tirefitlab.com/learn/load-index/)

### Useful tools

[Tire size calculator](https://tirefitlab.com/tire-size-calculator/)[Compare tire sizes](https://tirefitlab.com/compare/)