Tyre guide

Tyre sizing

Tyre sizes can be shown in several ways. You will find your tyre size on the sidewall of your existing tyre, and it is important to make sure the details match before placing an order. Agricultural and ATV tyres come in a range of configurations and are listed in both metric and imperial formats.

Tyre construction types

The symbol before the rim diameter tells you the tyre's construction type.

• Crossply (shown as a dash, for example 25x8.00-12) — woven plies of material are built into the tyre, each at a slightly different angle.
• Radial (shown as R, for example 165/80R13) — a radial belt runs around the circumference of the tyre.

Always run tyres of the same construction type on the same axle or machine. Mixing types can give unpredictable handling and cause premature wear.

What do the markings on a tyre mean?

If you are wondering what all the numbers and letters on a sidewall mean, you are not alone. There is more to choosing a tyre than size alone. Tyres are made and tested to work within set limits, and the sidewall markings all help you choose the correct tyre for the job.

Maximum inflation pressure

The maximum inflation pressure (PSI) is the highest pressure that should be used to seat the beads of the tyre onto the rim. Agricultural, turf and ATV tyres are notoriously difficult to fit, and it can be tempting to inflate far beyond the maximum pressure marked on the sidewall. This can cause premature failure of the tyre structure and should be avoided at all costs. Always use the correct tyre lubricant and fitting equipment for safe, reliable operation.

Operating pressure

The operating pressure (PSI) is the optimal pressure to run the tyre at. If the tyre has a load/speed index, the operating pressure is where the maximum rated load and speed are achieved. Inflating beyond the recommended operating pressure will not increase carrying capacity or performance. This differs from maximum inflation pressure: operating pressure is what the tyre is designed and tested to run at for best performance.

Load index

The load index is the maximum load a tyre can safely carry at the recommended operating pressure. Manufacturers design tyres to work within set weight limits and award a load rating once the tyre passes testing. Using the correct load rating matters: fitting a tyre that does not meet the demands of the job can lead to premature wear, damage or failure. The index is a number that represents the maximum designed and tested weight, not the weight itself.

Speed rating

The speed rating is the maximum speed a tyre is designed to run at. Like the load index, it is an important part of any tyre purchase: the correct tyre means better operation, less downtime and better overall value. Speed ratings are shown as a letter, from A1 (3 mph) up to Z (over 149 mph). Some tyres carry no speed marking, but most e-marked tyres show the load/speed index, usually clearly marked on at least one sidewall.

Ply ratings (PLY or P.R.)

Ply ratings were once the standard way to show the overall strength of a tyre, based on how many plies (layers) of rubber made up its construction. A 6-ply tyre was built from six layers. Modern materials and construction methods now add strength beyond rubber plies, so the ply rating is less about the actual number of layers and more about showing the strength as if the tyre had that many plies. It remains useful for judging overall strength and puncture resistance. Some tyres use a star symbol instead: one star equals two plies, so a 3-star rating is equivalent to 6-ply.

Mud and snow (M+S marking)

Some tyres are tested and marked as suitable for mud and snow or winter use. The M+S mark shows that the tyre will perform better in those conditions.

E-marking

An E-mark (or e-number) shows that a tyre has been manufactured to a specific EU standard and recorded for use in the EU. All tyres used on PLG registered vehicles, including all-terrain and utility vehicles, must be e-marked to be classed as roadworthy and legal. E-marks are also required to pass an MOT test in the UK for PLG registered vehicles.

DOT codes

DOT codes identify when a tyre was made. The code is usually four digits, split into two pairs: for example "1620" means the tyre was made in the 16th week of 2020. This can be useful in applications such as trailers and caravans.

Tyre load index chart

A tyre load index shows the weight a tyre can carry when fitted to manufacturer recommendations and pressures. Some tyres have multiple load capacities at different speeds and pressures, so always refer to the manufacturer guidelines and make sure the load and speed index suits your machine. Below are the most common load index marks found on tyre sidewalls.

Tyre speed rating chart

A tyre speed rating shows the speed a tyre can achieve when fitted to manufacturer recommendations and pressures. It is often combined with a load rating to show the weight a tyre can carry up to a given speed. Below are the most common speed rating marks found on tyre sidewalls.

Load and speed index

A load/speed index is usually found on the sidewall and is what manufacturers have tested and rated for maximum load and speed. Choosing the correct load/speed index for your vehicle matters: it is usually safe to increase it, but manufacturer guidelines should always be followed.

A load/speed index gives the weight and speed a tyre is capable of. For example, 116D indicates the tyre can carry 1,250 kg up to 40 mph. In agricultural tyres the load/speed index can change so that a vehicle travelling at a lower speed may carry more weight, so it is always worth checking the manufacturer's specifications.