Wheel fitment
Rim Width Range
A tire size is designed for a band of rim widths, not a single one. Mount it outside that band and the tread stops sitting flat on the road. This gives the measuring rim, the approved range, and what the section width actually becomes at each width in between.
The rim width a tire is designed for scales with its section width and profile.
Typical approved rim width for 225/45R17
7″ – 8.5″
The measuring rim — the width the tire's published dimensions are quoted on — is 7.5″. Section width is quoted at that width and moves by about 5 mm for every half inch of rim width either side of it.
| Rim width | Section width | Change | Note |
|---|---|---|---|
| 7″ | 220 mm | −5 mm | Narrower: more sidewall bulge, softer response |
| 7.5″ | 225 mm | — | Measuring rim — published dimensions quoted here |
| 8″ | 230 mm | +5 mm | Wider: sidewall pulled straighter, tread flatter on the road |
| 8.5″ | 235 mm | +10 mm | Wider: sidewall pulled straighter, tread flatter on the road |
What is a measuring rim?
Every published tire dimension — section width, overall diameter, revolutions per mile — is quoted with the tire mounted on one specific rim width, defined by the tire and rim standards organisations. That width is the measuring rim. It is the reference the whole specification hangs on, and it is why the same tire measures differently on two different wheels.
On a metric size the measuring rim is roughly 70% of the section width on a tall 75-series tire and climbs to about 88% on a 35-series, because a low-profile tire needs its sidewalls standing much more upright to work at all. That relationship is what the calculator above uses.
Flotation sizes work differently and are handled separately. Their published measuring rim is set by section width alone — exactly 2.5 inches under it, so 10.50 measures on an 8″ rim and 12.50 on a 10″ — and it does not move with height. A 33x12.50R15 and a 35x12.50R15 share the same measuring rim, which is why the aspect-ratio curve above is not applied to them.
Verified against published measuring rims
The model is derived, not licensed, so it is worth showing what it is checked against. These are published figures; the calculator reproduces every measuring rim below exactly, and all 11cases are asserted in the site's test suite.
| Size | Published measuring rim | Derived here | Published range | Range derived here |
|---|---|---|---|---|
| 235/75R15 | 6.5″ | 6.5″ | 6″–8″ | 6″–7.5″ |
| 265/70R17 | 8″ | 8″ | 7″–9″ | 7.5″–9″ |
| 195/65R15 | 6″ | 6″ | 5.5″–7″ | 5.5″–7″ |
| 215/60R16 | 6.5″ | 6.5″ | 6″–7.5″ | 6″–7.5″ |
| 205/55R16 | 6.5″ | 6.5″ | 5.5″–7.5″ | 6″–7.5″ |
| 225/45R17 | 7.5″ | 7.5″ | 7″–8.5″ | 7″–8.5″ |
| 245/40R18 | 8.5″ | 8.5″ | 8″–9.5″ | 8″–9.5″ |
| 275/40R20 | 9.5″ | 9.5″ | 9″–11″ | 9″–10.5″ |
| 31x10.50R15 | 8″ | 8″ | 7″–9″ | 7″–9″ |
| 33x12.50R15 | 10″ | 10″ | 8.5″–11″ | 9″–11″ |
| 35x12.50R17 | 10″ | 10″ | 8.5″–11″ | 9″–11″ |
Read this before you buy wheels
Common questions about rim width
- What rim width does a tire size need?
- Every size has a measuring rim — the width its published dimensions are quoted on — and an approved range around it, typically half an inch narrower to an inch wider. A 225/45R17 measures on a 7.5 inch rim and is approved from 7.0 to 8.5.
- What happens if the rim is too narrow?
- The sidewalls pull inward and bulge, the tread crowns so the center wears first, and steering response softens because the sidewall has to deflect further before the tread loads up. Too wide is the mirror image: the tread flattens, the shoulders take the wear, and the sidewall loses the compliance that absorbs impacts.
- How much does section width change with rim width?
- About 5 mm for every half inch of rim width. A 225 section tire on a rim one inch wider than its measuring rim will measure roughly 235 mm across — which matters when you are working out clearance to the strut.
- Is the rim width range the same for every brand?
- Almost, but not exactly. The range comes from tire and rim standards bodies, so it is consistent within a size, but individual manufacturers occasionally publish a narrower approved range for a specific model. The tire's own data sheet always wins.
Changing wheel diameter as well as width? The plus sizing calculator finds the sizes that keep your rolling diameter while the wheel grows.
Other tire maths on this site
- tire size calculatorRead any tire size code and get overall diameter, sidewall height, circumference and revolutions per mile, with the arithmetic shown.
- tire size comparisonPut two sizes side by side and see the difference in diameter, width, sidewall, ground clearance and speedometer reading.
- tire size chartEvery common size sorted by wheel diameter, with overall diameter in inches and millimeters and revolutions per mile.
- tire diameter calculatorOverall diameter, radius and rolling circumference from the size code, and the reverse — the sizes that produce a diameter you want.
- speedometer error calculatorHow far out your speedometer and odometer read after a tire size change, at any indicated speed.
- plus sizing calculatorLarger wheel, same rolling diameter. Plus one and plus two candidates ranked by how close they hold the original diameter.
- gear ratio calculatorThe axle ratio that restores stock engine rpm after a tire diameter change, plus cruising rpm at any speed.
- metric to inch tire conversionConvert a metric size such as 285/70R17 to its inch equivalent, and an inch size such as 33x12.50R17 back to metric.
- tire size meaningWhat every character in 265/70R17 stands for, digit by digit, including the load index and speed rating.