How to understand & read your tyres

Knowing how to interpret the letters and numbers on the side walls of your tyres is essential when it comes to changing them.

Although you can call into any of our branches for us to do this for you, it’s a good idea to learn how to find your tyre size – even if it’s just to compare tyre prices online and get the very best deal.
Tyres are an extremely important aspect to ensuring our vehicles are as safe as possible when on the roads. All tyres have writing and numbers imprinted on the sidewall which tell you everything from what make of tyre it is, to the size and even the date it was manufactured. Though many of the markings on your tyres are unnecessary information for you, as the driver, other markings are essential knowledge when it comes to replacing your tyres. Learning what information is important to you and how to recognise it on your tyre will make tyres a whole lot easier to understand.

The main piece of information you need to recognise and understand on your tyres is the tyre size, load rating and speed rating.

Width​

Tyre Width

225/40R18 92Y

The first three digits on the sidewall of your tyre show the width in millimetres.

For example, this means that if the tyres first three digital have been marked ‘225’, the tyre will then measure exactly 225mm across its tread from one sidewall to the other.

Profile

Tyre Profile

225/40R18 92Y

The fourth and fifth digit, in this case ‘40’, is located after the width digits. These show the height of the tyre as a percentage of the tyre width. In this case, the profile, sometimes known as the “aspect ratio” is ‘40’, which therefore means that the height of the tyre is 40% of the tyre width.

Radial

Tyre Construction | Radial

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Tyres that have been marked with the letter ‘R’ after the aspect ratio digits are radial tyres. This means they have been constructed with cord plies of which are positioned forward at a 90 degree angle. This allows the tyre to have additional strength, and most tyres that are manufactured in the present day are created this way.

Rim Diameter

Rim Diameter

225/40R18 92Y

The two digits located after the R (on a radial tyre) show the size of the wheel rim that the tyre can fit onto. It also shows the diameter of the tyre from each bead that sits on the edge of the wheel. In this case, the tyre is marked ‘18’, which tells the driver that it will fit onto a 18 inch wheel rim.

Load Index

Load Index | Load Rating

225/40R18 92Y

Located after the two digits that represent the diameter of the tyre are two more digits, in this case 92. These numbers can help you to figure out the maximum carrying capacity of your tyre in kilograms. In this example, the number 92 tells you that the tyre can carry 1260kg. It’s important to fit the correct load rating to your vehicle, especially if you have a heavy estate car, small van or 4×4.

Speed rating

Speed Rating

225/40R18 92Y

The final digital in this number and letter sequence can tell you the speed rating of your tyres. This allows you to know the maximum speed for a tyre that is correctly inflated and under load.  If you don’t know what speed rating your vehicle needs, check your handbook.

It’s important to remember that although you’ll never drive as fast as your speed rating allows, you should always fit the correct speed rated tyre for your vehicle. You can fit a higher rating but never a lower rating. Failure to fit an appropriate rating could, in the event of an accident, lead to your insurance being invalidated.

Tread wear indicators

Tread Wear Indicators (TWI)

Although TWI’s aren’t anything to do with your tyre size and specifications, it’s a very important aspect of your tyre markings and vital to tyre safety.  These little markers are located in the tread of your tyre and indicate when your tyre has reached the legal limit of 1.6mm. It’s important you check this regularly to ensure your tyres tread depth isn’t below the legal limit of 1.6mm.