Tyre width vs rim width
They are measured differently and they are not interchangeable. Tyre width is a section width in millimetres. Rim width is a bead-seat measurement in inches. One tyre width fits a range of rim widths.
The relationship in one line
A passenger tyre is normally approved for rims roughly 0.68 to 0.88 times its section width in inches, with the ideal around 0.76. A 225 mm tyre is 8.9 inches wide, so the usable range is about 6½" to 7½", ideal near 7".
Rim width changes the tyre's real width
Section width is quoted at a reference rim. Mount the same tyre on a wider rim and the casing is pulled outward: real section width grows by roughly 5 mm for every half inch of extra rim. Two cars with the same tyre size can therefore have measurably different tyre widths.
This is the detail that catches people out on clearance. Going from a 7" to an 8" rim with the same tyre adds about 10 mm of section width, half of it on each side, before offset is even considered.
What each direction does
| Narrower rim, same tyre | Wider rim, same tyre | |
|---|---|---|
| Tread profile | Crowned — centre contacts first | Flatter — shoulders loaded more |
| Sidewall | Bulges outward, more flex | Pulled taut, stiffer |
| Steering response | Softer, vaguer turn-in | Sharper, more direct |
| Ride comfort | Better absorption | Harsher over sharp edges |
| Kerb and pothole risk | Lower — sidewall protects the rim | Higher — rim sits closer to the edge |
Related
How are these fitment figures calculated?
Fitment North calculates declared wheel and tyre geometry from the dimensions shown on the page, using the same tested method as the wheel-offset and tyre-size calculator. Tyre diameter uses rim diameter plus two calculated sidewalls; offset comparisons split width changes across inner and outer edges before applying the offset difference. Results are measurement aids, not vehicle approval. Confirm ratings, clearances and manufacturer guidance before fitting parts.