Tyre Sizes & Markings, Explained

Every marking on a tyre sidewall means something. Read together, they tell you the tyre's size, how much weight it can carry, how fast it is built to go, the conditions it suits, and even the car it was designed for. This page is the map to all of it, with a plain-English guide to each marking, so you can read your own tyres with confidence.

A tyre's sidewall, decoded

Most car tyres on UK roads carry a code that reads in the same order. A common example is 205/55 R16 91V XL:

Read left to right, that tyre is 205mm wide, has a sidewall 55% of that width, uses radial construction, fits a 16-inch wheel, carries up to 615kg, is rated to 149mph and has a reinforced Extra Load casing. Once each part makes sense, working through any tyre size stops being guesswork.

The size: width, profile, construction and wheel

The first block of the code is the size itself. The width is the simplest part, the tread width in millimetres, measured across the tyre. The profile, or aspect ratio, is the part drivers most often get wrong: it is the sidewall height written as a percentage of the width, not a figure in millimetres, so the same number means a taller sidewall on a wider tyre.

After those comes a letter for the construction, almost always R for radial on modern cars, with ZR appearing on some high-speed tyres and older bias-ply designs now rare. The final number in the size is the wheel diameter in inches, which must match the wheel exactly. Each of these has a dedicated guide that goes further, with worked examples.

What a tyre can carry, and how fast

The numbers and letter that follow the size set the tyre's limits. The load index is a code that maps to a maximum weight per tyre on a standard chart, and the speed rating is a letter that maps to a maximum speed. They are deliberately written as short codes rather than the raw figures, which is why a full chart is needed to read them.

The safety rule behind both is simple: a replacement tyre should never carry a lower load index or a lower speed rating than the car maker specifies, because both are matched to the weight and performance of the car. The complete load index and speed rating charts live in their own guides.

Extra Load, reinforced and commercial casings

Some tyres carry an XL or Reinforced marking. This signals a stronger casing rated to carry more weight, usually at a higher inflation pressure, and it is common on heavier cars, electric vehicles and people carriers. Vans and light commercials use a C (Commercial) marking instead, often alongside a ply rating. Fitting a standard tyre where the car needs Extra Load can leave it under-rated for the weight it carries.

Markings for winter and wet weather

Two markings deal with cold-weather grip. M+S (mud and snow) is self-declared by the manufacturer with no test behind it, so on its own it proves very little. The three-peak mountain snowflake (3PMSF) is different: a tyre only earns it after passing an independent snow-traction test, which makes it the marking that genuinely shows winter capability. All-season tyres typically carry both, while dedicated winter tyres always carry the snowflake.

Markings for age and wear

A tyre also records its own age and condition. The date code, part of the DOT marking, gives the week and year a tyre was made, which is how an old or aged tyre is spotted even when the tread looks healthy. The small bars set into the tread grooves are tread wear indicators (TWI); once the tread wears down level with them, the tyre has reached 1.6mm, the UK legal minimum.

Markings for direction and fitment

Some tyres only work fitted one way round. Directional tyres have a rotation arrow on the sidewall that must point in the direction the wheel turns. Asymmetric tyres have different inner and outer halves and are marked Outside and Inside, so the correct face must point outward. Fitted the wrong way, both lose a large part of their wet-weather performance.

Construction and approval markings

A cluster of smaller codes describes how a tyre is built and who it was built for. Run-flat tyres carry maker-specific codes such as RFT, ROF, SSR or DSST. TL marks a tubeless tyre. Small carmaker approval marks show a tyre was tuned for a specific manufacturer, a star for BMW, MO for Mercedes-Benz, AO for Audi, N for Porsche. FR or MFS marks a rim protector, and the coloured paint dots on a new tyre are fitting references rather than anything a driver needs to act on.

The tyre label

Separate from the markings moulded into the tyre, every new tyre comes with a label rating it for fuel efficiency, wet grip and external noise. It is the quickest way to compare tyres on safety and running cost when buying. Reading the tyre label, and what each rating means, is covered in its own set of guides.

Van, 4x4 and alternative sizes

Not every tyre uses the standard car size format. Van and commercial tyres add a C to the size and carry a dual load index; many 4x4s use ordinary metric sizes, but some carry an LT light-truck marking or imperial flotation sizes such as 31x10.50 R15; and 4x4 tread is described as highway, all-terrain or mud-terrain. Van and commercial markings, 4x4 and SUV sizing, flotation sizes and all-terrain versus mud-terrain tread each have their own guide.

Changing tyre size

A different tyre size is possible within limits. The overall diameter should stay close to standard, within about 3%, the load index and speed rating must still be met, and tyres across an axle must match. Whether a tyre size can be changed, plus sizing, the effect on the speedometer and the rules on mixing tyres are covered in their own guides.

The EU tyre label

Separate from the sidewall is the EU tyre label, shown at the point of sale and on retailer websites rather than moulded into the rubber. It grades a tyre for fuel efficiency, wet grip and external noise, which makes it a useful way to compare tyres of the same size before buying.

Where to find a car's tyre size

The size is moulded onto the sidewall of every tyre already fitted. The car maker's recommended size, along with the correct pressures, load index and speed rating, also appears on a placard inside the driver's door, sometimes on the fuel filler flap, and in the handbook. In the UK, most tyre retailers can look the size up from a registration number. The placard figure is the safe baseline, as it is the specification the car was designed and approved around.

Changing tyre size

Fitting a different size is possible within limits, an approach known as plus-sizing, but it has to be done carefully. The load index and speed rating must still meet the car's requirements, and the overall rolling circumference needs to stay close to standard, or the speedometer reads incorrectly and handling and clearance can suffer. The rules around this, and how to work out an equivalent size, are covered in their own guide.

From the workshop: the marking drivers misread most is the middle number. A 55 is not 55 millimetres; it is a percentage, so on a wider tyre that same 55 gives a taller sidewall. Matching it to what is already on the car is the safest habit.

Common questions

What do the numbers on a tyre mean?

A size such as 205/55 R16 91V reads as the tread width in millimetres (205), the sidewall height as a percentage of that width (55), the construction (R for radial), the wheel diameter in inches (16), the load index (91, meaning 615kg per tyre) and the speed rating (V, meaning up to 149mph).

Where is the tyre size found?

It is moulded into the sidewall of the tyres already on the car. The recommended size is also on the placard inside the driver's door, sometimes on the fuel flap, and in the handbook, and most UK retailers can find it from a registration number.

Which marking is most often misread?

The profile, the middle number of the size. It is a percentage of the tyre's width rather than a measurement, so the same figure produces a taller sidewall as the tyre gets wider.

Does the load index or speed rating matter on a replacement?

Yes. Neither should drop below what the car maker specifies, as both relate directly to safety. The correct figures are listed on the door placard and in the handbook.

Sources and accuracy. The load index and speed rating figures on this page follow the standard industry reference tables used across the tyre trade, and are given as a general guide. The definitive values for any specific car are those on the placard inside the driver's door and moulded on the tyre's own sidewall. If anything here looks wrong, get in touch and we will check it and put it right.