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Tyre code

Car tyres have a long stretch of letters and numbers embossed on them, something like P215/65R15 95H M+S . Let’s decipher them together.

I very much enjoy codes and symbols that impart secret information – like Plimsoll lines on the sides of ships or resin identification codes (recycling numbers). But I know very little about cars. I didn’t get my driver’s licence until I was in my 30s! Because of this, I don’t know whether this interesting thing is obvious to everyone else on the planet or not. It was new to me, at least.

Car tyres (or tires for those of you in North America) typically have a series of numbers and letters embossed on their sides. These encode a lot of information about the tire, and the format of the whole thing follows an international standard that makes it easy to extract that information.

It begins with a letter. Well, it probably begins with the name of the company that manufactured the tyre, but we’re skipping over that brazen advertising to the important stuff. The letter tells you what kind of car this tyre is for: P for “passenger,” LT for “light truck,” and so on.

The next few symbols are numbers and they indicate the dimensions of the tyre: width in millimetres, height/width aspect ratio (like a TV!), and diameter of the wheel. That last one is measured in inches instead of a metric measurement – I have no idea why. There’s always a slash between the first two numbers and sometimes there’s a letter between the last two numbers: that letter indicates the tyre’s internal construction (it’s pretty much always R for “radial”).

The next section is an interesting one: it indicates how much of a load the tyre can carry, and how fast it can safely go with that load. You have to look up these codes to see what they actually represent: 95, for example, corresponds to 690kg, and H gives a top speed of 210km/h. The full chart is in the link below. Those letters for top speed are weird, by the way: apparently there exists a class of tyres (A1) that cannot safely go faster than 5km/h.

Finally, a bunch of other markings can convey extra information: M+S means they’re designed for “mud and snow,” RFT that they can be driven on even when flat, and so on. Car tyres sold in the United States have to include additional information about the place and time of manufacture – so you can always find out when your tyres were made.

 

Categories: Language Sciences Technology

The Generalist

I live in Auckland, New Zealand, and am curious about most things.

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