At first glance, the combination of letters, numbers and symbols embossed into the sidewall of a truck or bus tyre can be confusing. But this mix of characters contains some vital information that is critical to the safe and legal operation of your truck. It’s therefore important that tyre technicians understand what they mean. 

Tyre sizes are categorised using a universal code. If we look at a 315/70 R 22.5 tyre, the 315 represents the width of the tread in millimetres. In this case, the width is 315mm. The next two digits tell you how high the sidewall is, as a percentage of the tread width. So, on this tyre, the sidewall height is 70% of the tread width.

The ‘R’ tells you that the tyre is of radial construction, while the final digits correspond with the size of the wheel in inches. 

The service description, also known as a load index, often follows the tyre sizing information. This information is vital to the safe operation of your truck.  The first number indicates how much weight a tyre can carry in a single fitment, while the second, following a forward slash, corresponds to the maximum weight if the tyre is used in a twin fitment. These figures are both codes, so you’ll need to consult the manufacturer’s technical data guidelines to find out the weight and speed guidelines.

You’ll also see a letter. This represents the maximum operational speed for the tyre. Again, you’ll need to consult your technical data guidelines to confirm the maximum speed for the vehicle.

Your tyre may also carry more than one set of load index markings. These are known as the unique point and mean that the tyre can be run at different speeds and carrying capacities. You should not assume that a tyre has the same load index, just because it is the same size. For example, if your vehicle has a plated axle weight of 7.5 tonnes, you must use a tyre with a load index of 154 for it to be legal. 

To find out how old your tyres are, look for the letters ‘DOT’, followed by four digits in a box. The first two numbers are the week of manufacture, with the second representing the year. It is a legal requirement for these to be visible on your tyre as it is illegal to have a tyre older than 10 years on the front steer axle of an HGV. Tyres between 9 and 10 years old receive an advisory if subjected to official checks. 

The vast majority of Continental tyres feature the M + S designation. This stands for mud and snow and means that the tyre is suitable for slippery conditions. For the best performance on ice, mud and snow, you’ll want a tyre that carries the ‘3 Peak Mountain Snow Flake’ symbol.