Car Tyre Sizes
Most drivers are aware of the string of numbers and letters which are displayed on the side of their car tyres, but how many of us can honestly say we understand what they mean? In fact, reading these marks is not as difficult as you might think, and knowing how to interpret them may come in handy if you suffer a puncture, or are required to replace your tyres for an MOT. Since our customers contact Fast Tyre Service so frequently to have tyre marks explained, we thought an explanatory article was in order. Here’s a brief and concise guide to cracking the code.
Tyre Sizes
The first items of information which can be gleaned from sidewall codes relates to tyre sizes. There are hundreds of different car tyre sizes available on the market, as there are so many different types of vehicles with different requirements when it comes to rubber. But given all the possible variations, understanding the actual specification of the tyre you are looking at is quite simple.
The first three-digit number of the code is the tyre width in millimeters. This is the distance between the outside wall and the inside wall, and is the most basic measurement on which everything else depends. Following a slash mark is a two-digit number, which is the next most important measurement.
Here’s the technical bit; it isn’t vital to understand this when replacing your tyres, so skip to the next paragraph if you prefer. The second number is the tyre’s height from the wheel rim, expressed as a percentage of its width. These values are known in the trade as tyre aspect ratios, and make it easy for a fitter to locate the right tyre profile for a given size of wheel. Aspect ratios are used because a tyre rim size will change, as the surface of the tyre is worn down by contact with the road.
The important thing to know about these measurements is that tyres on the same axle must always match, and it’s best to have the same spec on all four corners in order to remain safe and legal. When ordering a replacement item from your supplier, always be sure to quote the sidewall code in full in order to receive the correct size.
Tyre Speed Rating
The letter following the numbers relates to the tyre’s internal structure. Since tyres are flexible, two models which look to be a similar shape and size may behave very differently when under load. Once again, make sure each new tyre fitted matches the one on the other side, whether it is marked ‘R’ for radial, ‘D’ for diagonal, or some other letter.
The two-digit number directly after the single letter is the final physical measurement provided, and gives the car wheel diameter in inches. It’s hard to get this one wrong; if it doesn’t match, the technician will be unable to fit the tyre to your wheel.
The number and letter which follow are properties which will not effect how the new tyre fits to your car, but will impact how it can be used. The number is a load index, and relates to how much weight the tyre can support. The letter is a tyre speed rating, which is the maximum speed at which the tyre can be used. Obviously, if either of these values are lower on your new tyre than on the one it replaced, it may impact how you use your car. That’s why it’s always best to have replacements supplied and fitted by a specialist like Fast Tyre Service of Brighton.
Any other markings after those described will represent additional information, such as manufacturing dates or safety codes. Customers can contact us for help in deciphering any unknown markings.
