Reading 225/45 R18 91W
| Part | What it is | Definition |
|---|---|---|
| 225 | Nominal section width in millimetres | UN R30, para. 2.17.1.1 |
| 45 | Nominal aspect ratio: section height as a percentage of the section width | UN R30, para. 2.15 |
| R | Radial construction: ply cords at roughly 90° to the tread centre line, stabilised by a circumferential belt | UN R30, para. 2.3.3 |
| 18 | Nominal rim diameter, given as a code in inches | UN R30, paras. 2.17.1.3 and 2.18 |
| 91 | Load-capacity index: 615 kg per tire | UN R30, Annex IV |
| W | Speed category symbol: 270 km/h | UN R30, para. 2.29.1 |
225/45 R18 is used across RideVector purely as an illustration. It is not a recommendation for any vehicle.
Definitions
Section widthThe tire’s nominal width, expressed in millimetres. Numbers below 100 in the rim position are inch codes; numbers above 100 are millimetres.
Aspect ratioThe section height divided by the nominal section width, multiplied by 100. A 225/45 tire has a sidewall of roughly 101 mm; a 225/40 tire on the same rim is shorter.
Load indexA number associated with the reference mass a tire can carry under the conditions of use specified by the manufacturer. The code-to-kilogram table is in the load index guide.
Speed symbolA letter expressing the maximum speed the tire can sustain. The full table is in the speed rating guide.
Other sidewall markings
| Marking | Meaning | Who defines it |
|---|---|---|
| REINFORCED / EXTRA LOAD | A carcass more resistant than the standard tire of the same size, so a higher load index. “XL” is the industry abbreviation; the regulation uses the two words. | UN R30, paras. 2.3.4 and 3.1.8 |
| M+S | Declares a “snow tire” by tread pattern and structure. No performance test is attached to the inscription. | UN R30, paras. 2.2 and 3.1.5; manufacturer declaration |
| 3PMSF (Alpine symbol) | Three-peak mountain with snowflake. Only permitted on a tire that passed the snow-grip test of UN R117 and is classified for severe snow conditions. | UN R117, paras. 4.2.6 and 6.4 |
| TWI | Tread-wear indicators: projections inside the grooves that show how far the tread has worn. | UN R30, para. 2.27 |
| Four-digit date | Week and year of manufacture, for example 0517 for week 5 of 2017. | UN R30, para. 3.1.9 |
| DOT | United States certification symbol that precedes the US Tire Identification Number. Not an EU requirement; the EU mark is the “E” approval mark. | 49 CFR § 574.5 |
| MO, ★, N0, AO and similar | Original-equipment codes: the tire maker built and the vehicle maker approved this tire for specific models. They mean approval, not a quality grade. | Vehicle manufacturers; documented by tire makers |
Why the size alone does not settle fitment
Two tires with the same size string can differ in load index, speed symbol, reinforcement and seasonal marking. A vehicle’s approved combinations list all of these, and the tire has to meet the load and speed requirements set for the vehicle. That is the subject of how tire fitment works.
Sources
- UN Regulation No 30 — Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers (Official Journal L 201, 30 July 2008)
- UN Regulation No 117 — Tyres with regard to rolling sound emissions, adhesion on wet surfaces and rolling resistance (Official Journal L 218, 12 August 2016)
- Michelin — Tyre markings explained
- Continental — Tire markings
- Continental — Tire age and the date code
- Pirelli — Custom-made tyre markings (OE codes)
- Pirelli — How to read the tyre DOT code
- 49 CFR § 574.5 — Tire identification requirements (United States)
All sources accessed 8 September 2026. UN Regulations are quoted from their Official Journal publications on EUR-Lex; later supplements may exist. This guide is general information, not advice for a specific vehicle: the vehicle documents and the applicable national rules take precedence.