In short. A tyre size is read from left to right. In 225/45 R17 91W, 225 is the width of the tyre in millimetres; 45 is the aspect ratio, the height of the sidewall as a percentage of the width (here 101 mm); R means radial construction; 17 is the diameter of the wheel it fits, in inches; 91 is the load index, a code for 615 kg (1,356 lb) per tyre; and W is the speed symbol, 270 km/h (168 mph). The date of manufacture is the last four digits of the code that starts with DOT: 2324 means week 23 of 2024. When you replace a tyre, the load index and the speed symbol should be at least those the car's manufacturer lists, and the size should be one it lists for your car. To compare two sizes (diameter, sidewall, speedometer difference), use our tyre size calculator. "Tyre" is the British spelling of the American "tire".
Where to find your tyre size
- On the sidewall, in the largest characters: that is the size actually fitted.
- On the manufacturer's label, often at the edge of the driver's door or inside the fuel flap, with the recommended pressures.
- In the owner's manual.
Wheels carry a size too, stamped on them, such as 8.5J x 19 ET45. The 19 is the diameter the tyre must match; our guide to wheel offset explains the ET.
Reading the size, element by element
UN Regulation No. 30, the UN rule on car tyres that Europe applies, defines each part of the marking (Regulation No. 30):
| Marking | In the example | What it means |
|---|---|---|
| Width | 225 | Nominal width of the tyre, in millimetres (8.86 inches) |
| Aspect ratio | 45 | Sidewall height as a percentage of the width: 225 x 0.45 = 101.25 mm (3.99 inches) |
| Construction | R | Radial, the construction of nearly every car tyre today; ZR is used on tyres rated for more than 240 km/h; F in front of the diameter (RF17, say) marks a run-flat |
| Wheel diameter | 17 | Diameter of the wheel the tyre fits, in inches |
| Load index | 91 | 615 kg (1,356 lb) per tyre, see the table below |
| Speed symbol | W | 270 km/h (168 mph) at most, see the table below |
The load index and the speed symbol together are the "service description", placed after the size.
Working out the overall diameter. The wheel diameter in millimetres (an inch is exactly 25.4 mm) plus two sidewalls. For a 225/45 R17: 17 x 25.4 = 431.8 mm, plus 2 x 101.25 mm, makes 634.3 mm, or 24.97 inches. That is what the tyre size calculator does for any two sizes you want to compare.
North American sizes. A size may start with a letter: P for a passenger-car tyre (P225/50R17 94V), LT for a light-truck tyre. The US safety standard defines a light truck (LT) tire as one its maker designates as primarily for lightweight trucks, and requires a load range letter on it (49 CFR 571.139). The P and LT prefixes are not part of the European marking; if you are comparing tyres from the two systems, compare the load index and the load printed on each tyre rather than assuming they match.
The load index: the full table
The load index gives the mass a tyre can carry at its reference pressure. UN Regulation No. 30 defines it as a number associated with a reference mass the tyre can carry when used as the manufacturer specifies (§ 2.28).
| Index | kg | lb | Index | kg | lb |
|---|---|---|---|---|---|
| 60 | 250 | 551 | 93 | 650 | 1,433 |
| 61 | 257 | 567 | 94 | 670 | 1,477 |
| 62 | 265 | 584 | 95 | 690 | 1,521 |
| 63 | 272 | 600 | 96 | 710 | 1,565 |
| 64 | 280 | 617 | 97 | 730 | 1,609 |
| 65 | 290 | 639 | 98 | 750 | 1,653 |
| 66 | 300 | 661 | 99 | 775 | 1,709 |
| 67 | 307 | 677 | 100 | 800 | 1,764 |
| 68 | 315 | 694 | 101 | 825 | 1,819 |
| 69 | 325 | 717 | 102 | 850 | 1,874 |
| 70 | 335 | 739 | 103 | 875 | 1,929 |
| 71 | 345 | 761 | 104 | 900 | 1,984 |
| 72 | 355 | 783 | 105 | 925 | 2,039 |
| 73 | 365 | 805 | 106 | 950 | 2,094 |
| 74 | 375 | 827 | 107 | 975 | 2,150 |
| 75 | 387 | 853 | 108 | 1,000 | 2,205 |
| 76 | 400 | 882 | 109 | 1,030 | 2,271 |
| 77 | 412 | 908 | 110 | 1,060 | 2,337 |
| 78 | 425 | 937 | 111 | 1,090 | 2,403 |
| 79 | 437 | 963 | 112 | 1,120 | 2,469 |
| 80 | 450 | 992 | 113 | 1,150 | 2,535 |
| 81 | 462 | 1,019 | 114 | 1,180 | 2,601 |
| 82 | 475 | 1,047 | 115 | 1,215 | 2,679 |
| 83 | 487 | 1,074 | 116 | 1,250 | 2,756 |
| 84 | 500 | 1,102 | 117 | 1,285 | 2,833 |
| 85 | 515 | 1,135 | 118 | 1,320 | 2,910 |
| 86 | 530 | 1,168 | 119 | 1,360 | 2,998 |
| 87 | 545 | 1,202 | 120 | 1,400 | 3,086 |
| 88 | 560 | 1,235 | 121 | 1,450 | 3,197 |
| 89 | 580 | 1,279 | 122 | 1,500 | 3,307 |
| 90 | 600 | 1,323 | 123 | 1,550 | 3,417 |
| 91 | 615 | 1,356 | 124 | 1,600 | 3,527 |
| 92 | 630 | 1,389 | 125 | 1,650 | 3,638 |
Indices 60 to 120: Annex IV of Regulation No. 30. Indices 121 to 125: the TNPF reading guide. Pounds are our conversion, rounded to the nearest pound.
Three points matter:
- It is the capacity of one tyre, not of the car. Each tyre carries its share of an axle, so the index your car's manufacturer lists for the axle is the figure to respect.
- It falls as the car's top speed rises. Regulation No. 30 lowers the maximum load rating when the car can go faster than the tyre's rating allows (§ 2.31). A V tyre on a car capable of 240 km/h (149 mph) may carry 91 per cent of its index load, a W tyre on a car capable of 270 km/h (168 mph) 85 per cent, and a Y tyre on a car capable of 300 km/h (186 mph) 85 per cent.
- XL means more load, at more pressure. A tyre marked XL, EXTRA LOAD or REINFORCED has a carcass that is more resistant than the standard tyre of the same size (§ 2.3.4), and a higher index. It reaches that index at the pressure given in its maker's load tables, not at the pressure of a standard tyre. Use the pressure for the tyre that is fitted.
In Great Britain, the MOT manual's guidance on checking load rating and speed rating is headed "Classes 5 and 7 only", which are larger passenger vehicles and goods vehicles, not ordinary cars. That is not a reason to fit a tyre rated below what your car's manufacturer lists. The Construction and Use Regulations bar the use of a tyre that is "unsuitable having regard to the use to which the motor vehicle … is being put" (regulation 27(1)(a)).
The speed symbol: the table
| Symbol | km/h | mph (rounded) |
|---|---|---|
| L | 120 | 75 |
| M | 130 | 81 |
| N | 140 | 87 |
| P | 150 | 93 |
| Q | 160 | 99 |
| R | 170 | 106 |
| S | 180 | 112 |
| T | 190 | 118 |
| U | 200 | 124 |
| H | 210 | 130 |
| V | 240 | 149 |
| W | 270 | 168 |
| Y | 300 | 186 |
Regulation No. 30, § 2.29.1, in km/h. The mph column is our conversion, rounded to the nearest mph.
The order is not alphabetical: H (210 km/h) sits between U and V. A tyre that is suitable for more than 300 km/h has ZR in its size and its service description in brackets, for example (95Y) (§ 3.1.4.1). For tyres rated W or Y the maker may also use ZR in place of R (§ 3.1.3.4), which is why a W or Y tyre can be marked either way.
The other markings on the sidewall
| Marking | What it means |
|---|---|
| XL, EXTRA LOAD, REINFORCED | Reinforced carcass, see above |
| F before the diameter (RF), RUN FLAT, and each maker's own initials (SSR at Continental) | A run-flat tyre: it can supply the basic tyre functions for at least 80 km at 80 km/h (50 mph) with no pressure (§ 2.3.7). Continental says its SSR tyres may only be used on cars with a tyre pressure monitoring system, because the driver would barely notice the loss; see our article on TPMS and the MOT |
| M+S, M.S, M&S | A "snow tyre" in the sense of the regulation, defined by its tread pattern and structure, designed to do better than an ordinary tyre in mud and fresh or melting snow (§ 2.2 and 3.1.5) |
| Three-peak mountain with snowflake (3PMSF) | The alpine symbol. A tyre may carry it only if it meets the snow performance requirements of UN Regulation No. 117, which compare it with a reference tyre in snow braking or snow acceleration |
| DOT and a code | The identification number of the tyre, ending in the date code, see below |
| E and a number in a circle | The tyre's approval mark under Regulation No. 30 (Annex II) |
| UTQG: treadwear, traction, temperature | US grading, see below |
The date code
The identification number starts with the letters DOT and ends in a group of four digits: the first two are the week of manufacture, the last two the year. UN Regulation No. 30 asks for the same date in the same form (§ 3.1.9); in the United States the identification number is defined in 49 CFR 574.5, whose date code must identify the week and year. The MOT manual gives the example of a code reading week 35 of 2016. 2324 means week 23 of 2024. The group may be moulded on one sidewall only: if you do not see it on one side, look at the other.
How old is too old? British law has no specific age limit for a car's tyres: the MOT manual's tyre-age check applies to vehicles with more than 8 passenger seats, and the ten-year limit in the Construction and Use Regulations is for buses, minibuses and heavier goods vehicles (regulation 27(1A)). Manufacturers give advice instead: Continental's data book recommends that tyres, the spare included, be removed from service if they were manufactured more than ten years before.
Tread depth and labels
- Tread. For cars, the MOT manual asks for primary grooves at least 1.6 mm deep across the central three-quarters of the breadth of the tread and around the entire circumference; the outer quarters of the tread may be bald.
- The label. Tyres sold in Great Britain carry a label showing fuel efficiency and wet grip, graded from A to G, and the external noise in decibels (GOV.UK). It is a label that comes with the tyre, not a moulding on the sidewall.
- UTQG, in the United States. The sidewall carries a treadwear number (a tyre graded 150 is expected to wear one and a half times as well as one graded 100 under the test conditions; grades are in multiples of 20), a traction grade for wet braking (AA, A, B or C, from best to worst) and a temperature resistance grade (A, B or C, A being the best) (49 CFR 575.104). These gradings do not apply to winter tyres, temporary spares or tyres on rims of 12 inches or less.
Which wheel width goes with which tyre
A tyre is not fitted to one wheel width but to a range. For each size, the ETRTO sets a measuring rim, on which the tyre is measured, and a range of permitted rims. Here is what Continental's technical data book gives, based on ETRTO values, for four common sizes:
| Tyre | Rim widths listed | Measuring rim |
|---|---|---|
| 205/55 R16 | 5½ to 7½ inches | 6½ inches |
| 225/45 R17 | 7 to 8½ inches | 7½ inches |
| 245/40 R18 | 8 to 9½ inches | 8½ inches |
| 255/35 R19 | 8½ to 10 inches | 9 inches |
The same tables show how the fitted tyre grows with the rim: each extra half inch of rim width adds about 5 mm to the width of the tyre (234, 239 and 244 mm for a 225/45 R17 XL on 7½, 8 and 8½ inch rims). Two conclusions follow. Read the range for your exact size in the maker's data rather than working it out from the tyre width. And a rim at either end of the range changes the shape of the sidewall, which is a question for whoever fits your tyres.
Changing size: what to check
- Same size and structure on an axle. In the MOT manual, tyres of different sizes on the same axle, or of different structure (radial and cross-ply), are major defects. A tyre fouling a part of the vehicle is also a major defect, if it is fouling at the time of the test.
- Load index and speed symbol at least those listed. Keep to what your car's manufacturer lists.
- Overall diameter close to the original. The speedometer counts wheel turns. Under UN Regulation No. 39 it must never show less than the true speed (§ 5.3). A tyre whose diameter is 4 per cent larger means a car that shows 80 mph is doing 83.2 mph, and one that shows 130 km/h is doing 135.2 km/h.
- A rim width in the range above.
How close is "close"? We have not found an official equivalence table for Great Britain; France publishes one, and it defines what a French tester accepts. Its four conditions for a replacement size are: a load index at least equal; a theoretical overall diameter within the ETRTO tolerance, which is plus or minus 3 per cent of the sidewall height; a width from 0 to 50 mm more than the original; and a wheel diameter from 0 to 3 inches more (TNPF). Applied to the two sidewalls of a 225/45 R17, the tolerance leaves roughly 6 mm on a 634 mm diameter, under 1 per cent. The "3 per cent of the diameter" that you will read on many websites, which would allow some 19 mm, does not come from that table. The table says itself that it is no guide to altering a car's fitment: it records what a test accepts, not what suits your car.
Frequently asked questions
What does 205/55 R16 91V mean?
A tyre 205 mm wide, with a sidewall 55 per cent of that width (113 mm), of radial construction, for a 16-inch wheel. It carries 615 kg (1,356 lb) at most (index 91) and is rated to 240 km/h (149 mph) (symbol V).
What is the difference between H and V?
The speed rating: 210 km/h (130 mph) for H, 240 km/h (149 mph) for V. On a car, the symbol should be at least that of the tyres the manufacturer lists.
How do I know how old a tyre is?
Read the last four digits of the DOT code on the sidewall: the week, then the year. 1023 is week 10 of 2023.
Where do I find the original tyre size for my car?
On the manufacturer's label (often at the driver's door or inside the fuel flap), in the owner's manual, or on the tyres themselves if they are still the ones the car left with.
Can I fit a higher load index than the original?
The index must not be lower than the manufacturer lists, and a higher one is acceptable. An XL tyre asks for the pressure its maker gives for it.
Can I fit a different size from the original?
Only one the car's manufacturer lists, or one close enough in diameter, load and width that your fitter, your insurer and your own check of the clearances agree. Use the tyre size calculator to compare diameters.
What wheel width suits my tyre?
One in the range listed for its exact size by the tyre maker: for example 7 to 8½ inches for a 225/45 R17 in Continental's data.
When the wheel changes
A new wheel set starts from the diameter and the width that carry a tyre your car is listed for, and from the PCD and the centre bore that decide whether the wheel can be fitted at all: see PCD, bolt pattern and centre bore. Our forged wheels start from the measurements of your car. Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards: see Onyx Forged and the catalogue.
Sources
- UN Regulation No. 30, revision 3, Official Journal L 201 of 30 July 2008: §§ 2.2, 2.3.4, 2.3.7, 2.28, 2.29.1, 2.31, 3.1 and Annex IV.
- UN Regulation No. 117, revision 4, copy published by the TNPF (in French); UN Regulation No. 39, § 5.3.
- GOV.UK, MOT inspection manual, section 5, 5.2.3; Construction and Use Regulations 1986, regulation 27; GOV.UK, tyre labelling rules.
- 49 CFR 574.5, 49 CFR 575.104 and 49 CFR 571.139.
- Continental, Technical Databook 2015-2016: permitted rims, measuring rims, tyre age.
- TNPF, equivalence table reading guide (in French), applicable from 1 November 2021.
All pages were read on 15 October 2026.
The Onyx Forged design office.