...how to readhow to read your tyre

How to read your tyre

You may feel bewildered by the array of letters and numbers that you see when you read the sidewall of your tyre.

However, the markings on the tyres do follow a universal logic and convey specific information that is very important for comparing products and making decisions about safety.

Once you can decipher the code, you’ll have almost all the details about your tyres that you’ll need. These markings are required to meet country/specific regulations and required by law.

How do you read a tyre code?

Your tyre code, which usually has the largest print of all characters on the sidewall of tyres (aside from the tyre and brand name), has seven main components.

The image below shows an example, which we then break down point by point.

1. Nominal Tyre Width

The three-digit number immediately following the vehicle type letter(s) tells you how wide your tyre is. This figure is in millimetres and is the nominal measurement from sidewall to sidewall at the tyre’s widest point excluding kerbing rib and raised lettering. In this example, the nominal section tyre width is 255mm.

2. Aspect Ratio

The aspect ratio indirectly tells you how tall your tyre is. It is expressed as a percentage of the tyre’s width. In the example code, the aspect ratio is 30, so the tyre’s height is 30 per cent of its width.

3. R - Internal Construction

The optional letter following the aspect ratio indicates how the body of the tyre is constructed. Standard construction for almost all consumer tyres is radial ply, which is indicated in this example by an R. Other construction types could be B (bias ply) or D (diagonal).

4. Rim Diameter Code

Following the tyre body construction letter is a two-digit number that tells you the diameter of the rims that the tyre is meant to be mounted on. This measurement is expressed in inches. In this example, the tyre is designed for 19-inch rims.

5. Load Index

The tyre load index indirectly states how much weight a tyre can bear. This number is not expressed directly in pounds or kilograms or any other unit for measuring weight. It is, rather, a numerical code that can be used along with a standardised reference table to look up the tyre’s actual safe load, in pounds, at maximum inflation pressure. In this example, the number 97 means that the tyre can withstand a load of 730KG.

6. Speed Rating

The tyre’s speed rating is also expressed indirectly, this time using a letter code. This letter can be used to look up the maximum speed that the tyre can safely withstand with a fully loaded vehicle (according to its load index rating). In this case, the letter V means that the tyre is rated for sustained speeds up to 149 miles per hour.

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Additional information on your tyres

Your tyre’s sidewall may also show the maximum air pressure, a traction rating and a treadwear rating, as well as the legally required DOT code, among other information. All of these aspects are explained in more detail below.

DOT code

The DOT tyre date code is a 7 to 13 character code that is legally required and is usually printed in smaller letters near the bead of the tyre (where the tyre meets the rim). Its first letters are DOT, which stands for “Department of Transportation” and indicates that the government agency has approved the tyre. The last four numbers of the DOT code are often the most useful to car owners because they indicate the age of the tyre. The first two digits of the last four numbers tell you the week the tyre was manufactured, and the last two the year. For instance, a tyre with a DOT code of 1119 was manufactured in the 11th week of 2019. This information is important because a tyre’s performance and safety diminishes with age, even if the tyre is not used, due to chemical breakdown. Other information included in the DOT code is place of manufacture, size and tyre type.

Maximum air pressure

Tyres may also contain text indicating the maximum air pressure to which they should be inflated. This pressure is stated in PSI (pounds per square inch). You should know that this pressure is not the same as the recommended pressure, which is specified in your operator’s manual or on the inside of your driver’s door. The maximum pressure is the most you can safely inflate your tyres but if you run your tyres at that pressure, they will most likely wear out more quickly and cause you braking and handling issues.

European ECE Type Approval

In the UK, the Motor Vehicle Tyres (Safety) Regulations 1994 stipulate that all tyres must have an approval mark to show that they conform to the European safety standards set out in ECE Regulation 30. The mark is made up of the letter E followed by a number indicating the country in which the tyre was approved – in the case of the UK, this would be E11.

Tread wear indicators

The minimum tread depth required by law for car tyres in the UK is 1.6mm across the central three-quarters of the tyre and around its full circumference. For your safety, it is important to bear in mind that the recommended minimum is 3mm.

You can check the tread depth of your tyres by locating the tread wear indicators on the sidewall of your tyre – sometimes indicated by the letters TWI but this can vary depending on the manufacturer. These blocks show the minimum tread depth of the tyre, so if the tyre tread is level with them, your tyres must be replaced straight away.

If you fall below the legal limit, not only could this have a severe impact on your safety and that of others, but you could also be faced with a £2,500 fine per tyre and three points on your driving licence.

Other markings on your tyre

Your tyre may also contain a letter indicating the load range. Unlike the load index, this single number does not refer to a specific amount of weight that the tyre can bear. Rather, it more generally specifies the toughness of the tyre and the amount of air pressure that it can hold. It can also be called the ply rating because tyres used to be built with different layers of cords and fabric called plies, and the more plies, the sturdier the tyre and the farther along in the alphabet the ply rating letter was. This type of construction is no longer used, but a tyre with a load range of C, for example, is the equivalent of a 6-ply tyre.

The tyre sidewall may also show information about the materials used to construct the tyre’s sidewall and body. Additionally, you may see the letters M+S, which stand for mud and snow. This means that the tyre has some ability to handle these slick conditions. However, a true winter tyre (which meets regulations for winter conditions) will also have a symbol of mountain peaks with snowflakes inside them.