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Reading the Sidewall of Your Tyres

Have you ever looked closely at the sidewall of your tyres and wondered what that long string of numbers and letters might mean? You are not alone, and the answer is satisfyingly simple. That code gives you the specifications of your tyres, letting your mechanic know all the important information about your tyres and letting them easily check that they are correct for your vehicle. Let us take a look at what all the letters and numbers actually mean.

Tyre Width

The first three digits refer to the width of the tyre in millimetres. This is the absolute width: a small proportion of that width will curve away into the sidewall, making the actual contact area slightly narrower. Tyre widths range from 125mm to upwards of 400mm but most fall between 165mm and 280mm for standard model cars.

Aspect Ratio: Sidewall Height

The following two digits are an expression of a percentage of the width of the tyre width, and this percentage gives the height of the sidewall of the tyre. For example, if the tyre width is 280mm, and the aspect ratio is 50% (this will appear on the tyre as 280/50) that means that the sidewall is 140mm high. The two numbers always appear together, separated by a forward slash, as shown in the example above.

Construction Method

The next notation on the tyre is almost always a single letter – these days, most commonly an R. This standard for radial. Other notations include D for diagonal and B for bias belt, while cross-ply tyres, now rather passe and old-fashioned, don’t have any notation in this place. Cross-ply tyres used to be the standard, so it was only new and unusual tyres that were denoted at first: today, they are quite rare having ‘aged out’ so to speak. If your tyres are getting on, this summer pamper your car with new tyres at affordable prices from Dartford Tyres.

Load Index

The next two digits refer to the load index which tells you the upper weight limit that the car can handle. This code is based on quite precise, scientific calculations involving the pressure of the tyre amongst other things, with the result that the individual numbers do not seem to relate in any intuitive way to the weight that the car can support, so you will have to look it up each time you need to know how much weight a certain car can support. For example, 89 means the car can carry 580kg, while 100 indicates a maximum weight of 800kg.

Speed Index

The final marker is usually a letter, and this lets you know how fast each tyre can be expected to go when operating at top speeds. For the UK, this is often well above the legal road limits, so it will seldom be of too much concern to drivers.

Now you know the secret of the tyre code and can order your new tyres with confidence!


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