Reading a tyre size
A common passenger-car marking is 225/45 R17 94Y. Each part communicates a different sizing or capability characteristic.
225nominal section width, mm
45aspect ratio
Rradial construction
17rim diameter, inches
94load index
Yspeed symbol
What the numbers mean
| Part | Meaning |
|---|---|
| 225 | Nominal tyre section width in millimetres. |
| 45 | Sidewall height expressed as a percentage of nominal section width. |
| R | Radial-ply construction. |
| 17 | Nominal rim diameter in inches. |
| 94 | Load index; convert using the applicable load-index table rather than treating it as kilograms. |
| Y | Speed symbol associated with a standardized test/category; replacement suitability still depends on vehicle approval and use. |
Other sidewall markings
XL commonly denotes extra-load/reinforced capability. M+S is a mud-and-snow marking. The 3PMSF mountain/snowflake mark identifies tyres meeting a defined snow-performance requirement. DOT/TIN-style codes can include manufacturing identification and a week/year date segment.
Replacement rule: use the vehicle placard/handbook and approved tyre specification. Size, construction, load capacity, speed capability, axle fitment and legal requirements all matter.