Quick Answer
- Tire stretch compares the rim width with the tire width. A 225/40R18 on a 9.5 inch rim has a 241.3 mm rim against a 225 mm tire: +7.2% stretch (calculated).
- Stretch is an enthusiast measure, not a manufacturer term. Continental permits 7.5 to 9 inch rims for a 225/40, so 9.5 inches is outside that range.
- The USTMA tire care manual says tires should only be mounted on approved rim widths, so use your tire maker's range, not a stretch percentage.
Enter the tire size and the rim width in inches to see the stretch percentage and whether the rim is in the range Continental permits for that tire.
The width between the rim flanges, as in 9.5J x 18.
Stretch
+7.2%
Rim minus tire
+16.3 mm
Continental permitted rims
Wider than Continental's permitted rims for 225/40 (7.5 to 9 in).
(9.5 x 25.4 - 225) / 225 = 7.2%
Stretch is an enthusiast measure (rim width in mm against the tire section width), not a manufacturer term, and the drawing is a schematic. The USTMA tire care manual says tires should only be mounted on approved rim widths, so use your tire maker's approved range, not the percentage.
Get a plain-English explanation of your results and what they mean for your vehicle.
Tire Stretch Chart: Stretch % by Tire Width and Rim Width
Stretch (positive) or pinch (negative) in percent for a tire width on a rim width. Calculated, not an approval: check your tire maker's approved rim range.
| Tire width | 6.5 in | 7 in | 7.5 in | 8 in | 8.5 in | 9 in | 9.5 in | 10 in | 10.5 in | 11 in |
|---|---|---|---|---|---|---|---|---|---|---|
| 195 mm | -15.3% | -8.8% | -2.3% | +4.2% | +10.7% | +17.2% | +23.7% | +30.3% | +36.8% | +43.3% |
| 205 mm | -19.5% | -13.3% | -7.1% | -0.9% | +5.3% | +11.5% | +17.7% | +23.9% | +30.1% | +36.3% |
| 215 mm | -23.2% | -17.3% | -11.4% | -5.5% | +0.4% | +6.3% | +12.2% | +18.1% | +24.0% | +30.0% |
| 225 mm | -26.6% | -21.0% | -15.3% | -9.7% | -4.0% | +1.6% | +7.2% | +12.9% | +18.5% | +24.2% |
| 235 mm | -29.7% | -24.3% | -18.9% | -13.5% | -8.1% | -2.7% | +2.7% | +8.1% | +13.5% | +18.9% |
| 245 mm | -32.6% | -27.4% | -22.2% | -17.1% | -11.9% | -6.7% | -1.5% | +3.7% | +8.9% | +14.0% |
| 255 mm | -35.3% | -30.3% | -25.3% | -20.3% | -15.3% | -10.4% | -5.4% | -0.4% | +4.6% | +9.6% |
| 265 mm | -37.7% | -32.9% | -28.1% | -23.3% | -18.5% | -13.7% | -8.9% | -4.2% | +0.6% | +5.4% |
| 275 mm | -40.0% | -35.3% | -30.7% | -26.1% | -21.5% | -16.9% | -12.3% | -7.6% | -3.0% | +1.6% |
| 285 mm | -42.1% | -37.6% | -33.2% | -28.7% | -24.2% | -19.8% | -15.3% | -10.9% | -6.4% | -2.0% |
| 295 mm | -44.0% | -39.7% | -35.4% | -31.1% | -26.8% | -22.5% | -18.2% | -13.9% | -9.6% | -5.3% |
| 305 mm | -45.9% | -41.7% | -37.5% | -33.4% | -29.2% | -25.0% | -20.9% | -16.7% | -12.6% | -8.4% |
How Much Stretch Do Continental's Permitted Rims Allow?
The rim widths Continental permits for common low-profile sizes (Tyre Databook 2025), and the stretch each end of the range corresponds to. Other makers can list a different range.
| Tire size | Permitted rims (in) | Stretch at narrowest | Stretch at widest |
|---|---|---|---|
| 205/40 | 7 to 8 | -13.3% | -0.9% |
| 215/40 | 7 to 8.5 | -17.3% | +0.4% |
| 225/40 | 7.5 to 9 | -15.3% | +1.6% |
| 225/35 | 7.5 to 9 | -15.3% | +1.6% |
| 235/40 | 8 to 9.5 | -13.5% | +2.7% |
| 235/35 | 8 to 9.5 | -13.5% | +2.7% |
| 245/40 | 8 to 9.5 | -17.1% | -1.5% |
| 245/35 | 8 to 9.5 | -17.1% | -1.5% |
| 255/35 | 8.5 to 10 | -15.3% | -0.4% |
| 265/35 | 9 to 10.5 | -13.7% | +0.6% |
| 265/30 | 9 to 10 | -13.7% | -4.2% |
| 275/30 | 9 to 10 | -16.9% | -7.6% |
| 285/30 | 9.5 to 10.5 | -15.3% | -6.4% |
| 295/30 | 10 to 11 | -13.9% | -5.3% |
| 305/30 | 10.5 to 11.5 | -12.6% | -4.2% |
For any size use the rim width calculator, and see the wheel offset and backspacing calculator and the plus sizing options calculator for the rest of a fitment.
What the Tire Industry Says About Rim Width and Stretch
| Source | Statement (paraphrased) |
|---|---|
| USTMA, Care and Service of Passenger and Light Truck Tires (2017) | Tires should only be mounted on approved rim widths; refer to the tire manufacturer's product literature for rim width specifications. Rim width is the measurement on the inside of the rim between the two flanges. |
| USTMA | Always use approved tire and rim combinations for rim diameter, width and contour. 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. |
| USTMA | Lower aspect ratio tires typically aid performance and handling but may provide a less comfortable ride, may wear more quickly and produce more noise, and are more susceptible to damage from road hazards and pothole or curb impact. |
| 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, and Continental does not approve any combination outside TRA, ETRTO or JATMA standards. |
Stretch percentage
(rim width in mm - tire width in mm) / tire width x 100. For 9.5 in on a 225 mm tire: (241.3 - 225) / 225 = +7.2%. Positive is stretch, negative is pinch.
Rim width in mm
Rim width in inches x 25.4. A 9.5 inch rim is 241.3 mm. Measure between the two flanges, as USTMA defines rim width.
Tire width
The first number in the size (225 in 225/40R18) is the nominal section width in mm on the tire maker's measuring rim. It changes with the rim you fit, which is why makers publish a range.
Permitted range
Look up your size in the tire maker's data book. The rim width calculator lists the Continental range for any size, and the table on this page shows the stretch that range corresponds to.
How to Use the Tire Stretch Calculator
- 1Enter the tire size, for example 225/40R18, and the width of the rim in inches.
- 2Read the stretch or pinch percentage and the millimetre difference, and look at the schematic.
- 3Check the Continental line: it says whether the rim is inside, narrower or wider than the range Continental permits for that size.
- 4Confirm the approved rim width for your exact tire with its maker, and check clearance before fitting.
Frequently Asked Questions
Sources: USTMA, Care and Service of Passenger and Light Truck Tires (2017); Continental Tyre Databook 2025 and Continental plus-sizing guidance. Stretch percentages are calculated. Confirm the approved rim width with your tire manufacturer and check clearance before fitting.