Tyre width explained
The first number on a tyre — 205 in 205/55R16 — is its section width in millimetres, measured at the widest point of the inflated tyre including the sidewall bulge.
Three widths, one tyre
Section width is the sidewall number. Tread width is the part touching the road, usually 15 to 25 mm narrower. Overall width includes raised lettering and rim protectors and can exceed section width by a few millimetres.
For clearance, section width is the one that matters, because the sidewall bulge is the widest point and the first thing to touch a strut or arch liner.
| Measurement | What it measures | Relative size | Use it for |
|---|---|---|---|
| Section width | Widest point of the inflated tyre, sidewall included | The sidewall number | Clearance |
| Tread width | The part actually contacting the road | 15–25 mm narrower | Grip and wear |
| Overall width | Includes lettering and rim protector ribs | A few mm wider | Tight arch checks |
Width changes more than grip
Going wider at the same aspect ratio also makes the tyre taller, because the aspect ratio is a percentage of the width. Going from 205/55R16 to 225/55R16 adds 20 mm of width but also 11 mm of sidewall, raising rolling diameter by around 2%. To go wider without getting taller, the aspect ratio has to come down — which is exactly what a plus-size does.
Each tyre size comparison shows both effects together.
Wider is not automatically better
A wider tyre puts more rubber down and usually improves dry grip and braking. It also increases the risk of aquaplaning, adds rolling resistance and unsprung weight, makes the steering heavier at parking speeds, and costs more to replace. Manufacturers pick OE width as a compromise, not a limit.
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.