Expert advice: what the digits and letters on your tyre sides mean

Tyre sides

If your car suffers one of these you’ll need the characters in the picture below

 

You may never have looked at the writing on your tyre sides. And if you have, there’s every chance you’ll think they’ve been written in another language. But strange as these codes may look, they’re important because if you have a puncture, or your tyres wear out, they give you all the information you need to choose a replacement.

If you look at the side of a tyre, you’ll see characters like 205/55 R16. This is the most basic information you’ll need to tell a retailer if you’re hunting around for new tyres. But other details are vital too. You must choose a load index that is right for your car. Use tyres with the wrong one and you could invalidate your insurance.

The speed rating is important as well. If you have the wrong speed rating and you suffer a tyre failure, you may not be covered by your insurer. You’ll be able to find the correct load index and speed rating for your car in its user manual. Here’s my guide to what the most important characters on your car’s tyres mean.

Tyre side

From the left going up are tyre width, profile height, construction, wheel diameter, load index and speed rating. On the right is date of manufacture

Tyre width

The first three digits signify the total width of the tyre in millimetres. This is how wide the tread is from side to side. The number will be bigger for a sportscar which has fatter tyres, smaller for an eco car with very narrow tyres.

Profile height

The fourth and fifth digits are what’s known in the trade as the aspect ratio. I prefer to think of this as the profile. This is the height of the tyre’s sidewall but it’s expressed as a percentage of the width. The bigger the tyre’s width number, usually the lower the profile number is. And frequently, the sportier the car, the lower this profile number.

Construction

Until the late 1940s, all tyres were crossply construction. This featured nylon chords or plies beneath the rubber lying at 55-degree angles to each other. Then Michelin came up with the radial tyre. They have the plies at right angles (90 degrees) to the direction of travel. This makes them longer lasting and better at absorbing bumps. The R signifies that tyres are radials; virtually every tyre is now.

Wheel diameter

The final two digits are the diameter of the wheel in inches. The majority of cars sit on 16 or 17-inch wheels. Usually, the sportier the car, the bigger the wheel.

Load index

Following the characters above, there are two more digits and a letter that are also important. The two digits are the load index. This is how much in kilograms each tyre can carry at its fastest speed. For a regular family car it is likely to be around 91. This equates to 615kg. As each tyre can carry that, it means the four tyres can support 2460kg between them. A regular family hatchback usually weighs somewhere between one and one and a half tonnes.

Speed rating

Every tyre has a speed rating. This is because tyres are tested to determine the maximum speed they can perform at. These start at L (75mph) and usually increase in 6 or 7mph increments to U which is 124mph. Oddly, this is then followed by H (130mph), V (149mph), Z (150mph plus) with W signifying 168mph and Y 186mph.

Date of manufacture

The other important numbers to look for are less straightforward to find: they’re how to age your tyre. This is displayed in the form of four digits. These are usually in a box (they might be preceded by some letters) and they’ll tell you when the tyre was made. This is in the form of week of the year and year. So 1514 is the 15th week of 2014. This is important because the older tyres get, the less effective they become. And if your tyres have cracks forming on the sidewalls they could be suffering structural damage. And that means it could be time to replace them, no matter how much tread they’ve got left on them.

self-driving carsNick Reid is head of automotive technology at Direct Line Group and is also a fellow of the Institute of the Automotive Industry

 

 

 

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