Quick Answer
- A tire fits a wheel when three things hold: the rim diameter matches the number after R, the rim width is inside the range the tire maker approves, and the tire clears the car.
- Example: a 225/40R18 on a 9.5J x 18 wheel matches in diameter, but Continental permits 7.5 to 9 in for that size, so the width is outside range (+7.2% calculated stretch).
- Tire makers publish approved rim widths rather than a stretch percentage. USTMA says to mount tires only on approved rim widths.
Enter the tire size and the wheel (width x diameter, for example 9.5x18) to check diameter, rim width and stretch.
225/40R18 on a 9.5J x 18 wheel
Rim diameter
The tire is marked for a 18 inch rim and the wheel is 18 inches.
Rim width
A 9.5 inch rim is wider than the 7.5 to 9 inch range Continental permits for 225/40.
Stretch
Rim 241.3 mm against a 225 mm tire is +7.2% (calculated, not a manufacturer measure).
The USTMA tire care manual says tires should only be mounted on approved rim widths and that clearance must be checked at all four wheel positions. Check load index and speed rating against your vehicle placard too.
Get a plain-English explanation of your results and what they mean for your vehicle.
Example Tire and Wheel Pairings
Results from the same checks the calculator runs. Width ranges are from the Continental Tyre Databook 2025; stretch is calculated.
| Tire | Wheel | Diameter | Continental range | Rim width | Stretch |
|---|---|---|---|---|---|
| 225/40R18 | 9.5J x 18 | Match | 7.5 to 9 in | Outside range | +7.2% |
| 225/40R18 | 8J x 18 | Match | 7.5 to 9 in | Inside range | -9.7% |
| 245/40R18 | 9.5J x 18 | Match | 8 to 9.5 in | Inside range | -1.5% |
| 255/35R19 | 8.5J x 19 | Match | 8.5 to 10 in | Inside range | -15.3% |
| 235/45R17 | 7.5J x 17 | Match | 7.5 to 9 in | Inside range | -18.9% |
| 205/55R16 | 6.5J x 16 | Match | 6 to 6.5 in | Inside range | -19.5% |
| 225/40R18 | 8J x 17 | Mismatch | 7.5 to 9 in | Inside range | -9.7% |
| 265/35R18 | 10J x 18 | Match | 9 to 10.5 in | Inside range | -4.2% |
To see the range for any size use the rim width calculator, and for the stretch drawn to scale see the tire stretch calculator.
What Tire Makers and USTMA Say About Fitting a Tire to a Rim
| Source | Statement (paraphrased) |
|---|---|
| USTMA, Care and Service of Passenger and Light Truck Tires (2017) | Check that the diameter designation of the tire and the rim are the same, for example a P235/75R16 tire goes on a 16 inch rim. Tires should only be mounted on approved rim widths listed in the tire maker's literature. |
| USTMA | Acceptable load index, load capacity or overall diameter do not imply acceptable vehicle clearance. Check clearance at all four wheel positions through full suspension travel and full steering lock. |
| Continental Tire, Plus-sizing guidance | Only use industry-approved tire size and rim width combinations. A rim that is too narrow or too wide affects handling, stability, wear and durability. |
| Continental Tyre Databook 2025 | Lists a permitted rim width range for each tire size, for example 7.5 to 9 inches for a 225/40. These ranges are what the width check on this page uses. |
1. Diameter
The number after R is the rim diameter in inches. A 225/40R18 needs an 18 inch rim. This must match exactly; there is no tolerance.
2. Width
Each tire size has an approved rim width range. The checker compares your rim with the Continental range for that size. Other makers can list slightly different ranges.
3. Stretch
Calculated as (rim mm - tire mm) / tire mm x 100. It describes the shape of the fit but is not a manufacturer limit.
4. Clearance and load
Offset, fender and suspension clearance and the load index are not checked here. Use the wheel offset calculator and the load index calculator for those.
Related: wheel offset calculator, load range and ply rating calculator.
How to Check If a Tire Fits a Rim
- 1Enter the tire size, for example 225/40R18, and the wheel as width x diameter, for example 9.5x18.
- 2Read the three checks: diameter, rim width against the Continental range, and stretch.
- 3To start from a wheel, open the Tires for a Rim Width tab and enter the rim width in inches.
- 4Confirm the approved rim width with your tire maker and check clearance before buying.