The sidewall,
decoded
Everything moulded into that line is a specification, and none of it is decorative. Here is what each field is, in the order it appears.
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·drawn to scale from the size designation — 1 px = 2.87 mm·REVIEWED ETRTO STANDARD
Field by field
- 225
- Section width, 225 millimetres. The tyre's width at its widest point when it is mounted and inflated — sidewall bulge included, not the tread. In inches that is 8.86″.
- 45
- Aspect ratio, 45%. The sidewall is 45% as tall as the tyre is wide, so 225 × 45 ÷ 100 = 101.3 mm. A lower number is a shorter sidewall and a stiffer ride.
- R17
- Radial construction, 17-inch rim. The R is the carcass type, not a size. The 17 is the wheel diameter the tyre beads onto — 431.8 mm — and it is the one dimension of the three that is still in inches.
- 91
- Load index — 615 kg per tyre. A code into a standard table, not a weight in itself. Four of them means the set is rated for 2,460 kg. Never fit below your vehicle's placard figure. Full table →
- V
- Speed symbol — 240 km/h. The speed the tyre is constructed to sustain, not a recommendation. Higher than your car needs is fine; lower is not. Full table →
The codes that are not the size
- DOT code. Four digits at the end — week and year of manufacture. 3223 is the 32nd week of 2023. Rubber ages whether it is driven on or not, which is why this matters on a "new" tyre that has been in a warehouse.
- Treadwear, traction, temperature. The UTQG block. Treadwear is a relative index against a reference tyre and is set by the manufacturer's own testing, so it compares within a brand far better than across brands.
- M+S, and the three-peak mountain snowflake. M+S is a tread-pattern claim the manufacturer makes about itself. The snowflake is a tested severe-snow certification. They are not the same thing and the difference is the whole winter-tyre argument.
- XL or Reinforced. A stronger casing rated to carry more at a higher pressure — it comes with its own load index, which is the figure that actually matters.
- The arrow, or "outside". Directional and asymmetric tyres only go on one way. Fitted backwards they lose most of their wet performance and keep all of their price.
Speed symbols in full
| Symbol | Maximum km/h | mph | Typically fitted to |
|---|---|---|---|
| L | 120 | 75 | Off-road and light-truck winter |
| M | 130 | 81 | Temporary spare |
| N | 140 | 87 | Temporary spare |
| P | 150 | 93 | Some light-truck |
| Q | 160 | 99 | Studless and studded winter |
| R | 170 | 106 | Heavy-duty light truck |
| S | 180 | 112 | Family saloons and vans |
| T | 190 | 118 | Family saloons and vans |
| U | 200 | 124 | Saloons and coupes |
| H | 210 | 130 | Sport saloons and coupes |
| V | 240 | 149 | Sports cars |
| W | 270 | 168 | Exotic sports cars |
| Y | 300 | 186 | Exotic sports cars |
| (Y) | 300 | 186 | Above 300 km/h — the brackets mean the rating is open-ended |
SOURCE·ETRTO / T&RA published speed-symbol table·REVIEWED ETRTO STANDARD
Sidewall codes — the questions that follow
What do the numbers on a tyre mean?
A size like 225/45R17 91V is four separate facts: 225 is the section width in millimetres, 45 is the sidewall height as a percentage of that width, R17 is radial construction on a 17-inch rim, 91 is the load index (615 kg per tyre) and V is the speed symbol (240 km/h).
What does the load index mean?
It is a code, not a weight. 91 does not mean 91 of anything — it is a row in a standard table, and that row says 615 kg. Fitting a tyre with a lower load index than your vehicle's placard specifies is the one change on this whole site that is unambiguously unsafe.
Can I fit a higher speed rating than my car needs?
Yes, and it is common — a V-rated tyre on a car that only needs T is fine. The reverse is not: the speed symbol is a maximum the tyre is constructed to sustain, and dropping below the vehicle's requirement can invalidate insurance as well as being a genuine limit.
What does the R in 225/45R17 stand for?
Radial — the way the carcass plies are laid, running from bead to bead across the tyre. It is a construction type, not a dimension. You will occasionally see D (diagonal, or bias-ply) or B (belted bias) on older and specialist tyres.
Put your own size in on the calculator, or read the load-index table. The geometry here is exact; whether a tyre fits YOUR car is not something geometry can answer. Arch clearance, suspension travel, offset, load rating and your vehicle's own placard all decide that, and none of them are in a size string. The ±3% rule quoted throughout is an industry convention, not a law. Confirm any change against the tyre placard on your door pillar and with a tyre fitter before you buy.