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If you’re new to the world of motoring or if, up until now, you’ve been one of those people who has changed tyres the lazy way (going to the mechanic and saying “Put another set on, mate.”), you might never have looked at the tyres closely. If you do crouch down beside them to wash them or whatever, you might notice a string of letters and numbers that look as though the cat has been walking on the computer keyboard. Something along the lines of 251hggggggggggggggggggggggggl (that wasn’t on the side of a tyre – that actually was the cat). Now, these numbers aren’t just put on there for the fun of it, to decorate the tyres or to give the blokes and blokesses who make and change the tyres something to look at. They’re to let you know important facts about your tyre.


Confession time: until quite recently, this writer was one of those lazy people when it came to changing tyres, and it was something that the other half took care of. I was more like Mr Bean in “Mr Bean’s Holiday”, where the word “Dunlop” on the tyre of Sabine’s Mini was about all he could recognize in a welter of gobbledegook. I had a code to learn and to decipher. The code isn’t that hard, really, once you have a sort of Rosetta Stone to help you learn the ancient secrets of tyre-oglyphics.


This code is used to decipher ISO (International Organisation for Standardisation) codes. As they’re international, you don’t have to get a new Rosetta Stone to decipher the tyreoglyphics for cars that have been imported. We’d all go mad if we had to do that and might even consider going back to bicycles, except they have codes on their tyres, too.


The first bit of the code is a letter. This indicates the sort of vehicle you should put the tyre on. The letter that most readers will have on their car tyres is P, which stands for passenger vehicle. However, if you have put on the space-saver tyre and haven’t bothered to change it to a real one after getting a flattie (bad idea but at least some manufacturers, such as Volvo, don’t compromise your safety for a bit of extra boot space and don’t give you these), you will see T for temporary, meaning that you’re only supposed to use it to limp to the mechanics. If you see ST, you’ve got a Special Trailer tyre and if you see LT, you’ve got a Light Truck tyre.


The second bit of the code is also fairly easy to understand. This consists of three letters in front of a slash (/). This gives you the tyre’s full width in millimetres, measured at the widest point. No worries there.


After the slash, it gets a wee bit harder and some bits are optional. First of all, you’ll get the tyre’s aspect ratio, which is a two-digit number, as it’s a percentage. The aspect ratio of a tyre is the sidewall height divided by the width. This number can be left out, in which case the aspect ratio is 82% (the % sign gets left off the tyreoglyphics).


Then you might get another letter that shows how the tyre has been made. This will give you an indication of how the tyre handles on the road. Here, B is for Bias Belt, D is for Diagonal and R is for Radial. If you don’t see a letter in this part of the tyreoglyphics, then the tyre is a good old cross-ply.


After this, you get the diameter of the tyre, which is very straightforward apart from one thing: it’s in inches. You’d think that in these days when nearly every country of the world except for the USA uses metrics for everything that we’d be using millimetres or centimetres to measure tyre diameters. However, like TV screens and certain parts of the male anatomy, tyre diameter sticks with inches, and in two cases out of three, the general rule is that the big ones are the luxurious versions. Take a look at any vehicle that has a luxury variant or a sporty upgrade on the bog-standard type: chances are that the posh version has bigger tyres. For example, the basic Opel Corsa has 15-inch tyres, the Opel Corsa Colour has 16-inch tyres and the sports upgrade has 17-inches.


Then you get into the really hard bits. After the diameter, you’ll find a two-digit code indicating the load index, which indicates the maximum load (weight) that the tyre can carry. However, the ISO haven’t done anything as sensible as making this number equal the weight in kilos or even pounds. A mathematician could probably find the formula that links a weight of 280 kg (or 620 pounds) to the figure of 64 and all the rest of it. But for the rest of us who aren’t maths professors, there’s nothing to do but to learn the codes or look them up. You can find load index tables online really easily: here’s one.


After the load index, you get the speed index, which is another code. The speed index tells you the maximum speed you can go when the tyre is carrying the maximum load. This is coded as a letter and is a bit easier to wrap your head around. At first glance, it looks easy, but there’s a few quirks. If you have a tyre with A on the side (unlikely these days), it’ll have a number after the letter: A1 means the maximum speed is 5 km/h, A2 means 10 km/h and so forth up to A8 meaning 40 km/h. So far, so good. B is 50 km/h and C is 60 km/h, but D isn’t what you would expect and is 65 km/h while E is 70 km/h. The rest of the alphabet progresses more or less normally, going up a letter for every 10 km/h, except that I has been left out because it looks like a one and the person who wrote this code decided that H, W and Y need to go where you don’t usually see them. H goes between U and V, and indicates a maximum speed of 210 km/h. W comes after Z and indicates 270 km/h and Y comes last of all and indicates 300 km/h. If you see a set of parentheses around the W or the Y, it means that you can go over these speeds, so (W) means you can go over 270 km/h and (Y) means you’re a race driver who can go over 300 km/h.


This just scratches the surface of tyreoglypics, and there are other odds and ends you can find on your tyre, including codes to say that this particular tyre is approved by, say, Mitsubishi and Toyota (MZ). But that thing with the letters and numbers with a slash in the middle is the really important one. Ask your mechanic about the rest.