Figuring out tyre’s vital statistics also vital for safety

These numbers and letters are a tyre's vital statistics but do you know what they mean?

These numbers and letters are a tyre's vital statistics but do you know what they mean? - Credit: PA

Ever wondered what all the numbers mean on the sides of your tyres? Wonder no more with this handy guide.

Beyond checking the pressure in your car's tyres now and then, how much attention do you really pay to these most vital of safety items? Learning a bit more about the only point of contact between the road and the rest of your car makes sense, and can also help you shop for the best deal when replacing rubber.

Almost all tyres fitted to modern cars use a radial construction rather than the crossply method found on most pre-1970s cars. With radial tyres, there are several numbers you can read on the sidewall and, at first, it can just look like a jumble of random digits.

However, as with all sorts of numeracy, a little effort to untangle the riddle is easily done and soon explains a lot about the tyres on your car. The 'R' in R16 tells us this is a radial tyre.

As an example, the car you drive might have '205 55 R16 91 W' moulded into the sidewall. A little like a supermodel, these are the tyre's vital statistics.

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First off, the '205' refers to the tyre's width in millimetres. When replacing a tyre, it's essential to use a tyre of the same width unless you have checked with the manufacturer that it's acceptable to use one of a different breadth. Swapping without any checks could lead to the tyre fouling the bodywork, wearing through and causing a potentially dangerous blowout. Also, fitting tyres of a different width to the same wheel can lead to problems with tread wear and grip.

Next up is the '55' on the tyre. This isn't anything to do with age. Instead, it's all about the sidewall's depth measured as a percentage of the width. While that might sound arcane, it's a very accurate way to make sure the tyre's size is in the right proportion for your car.

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So, in this example, a 55 sidewall equates to a tyre with a shoulder that's 55% of 205mm. This is also referred to as the aspect ratio. You can choose a tyre with a different aspect ratio, but remember this will also alter the gearing of the car and the speedo reading unless you are also changing the overall size of the wheels but keeping the overall diameter the same.

The size of the wheel is denoted by the '16' part of the R16 and let's us know the tyre must have a 16in diameter inner ring to fit the wheel. If it doesn't fit on the wheel snugly, it won't seal and hold air. There are no nearly-fits here – it either fits or it doesn't.

Now, the '91' is the load rating of the tyre, which is all about how much weight each tyre can safely support. In the case of a 91 tyre, it can take 615kg of load, so four tyres can cope with a 2,460kg maximum weight – but it also depends on how much weight is over each axle. As a rule, front-wheel-drive cars have more weight over the front wheels than the rear on level ground. The higher the number, the more weight the tyre can deal with, but it's good practice to leave a generous margin to make sure the tyre is not put through too much stress.

Lastly, the 'W' is the speed rating of the tyre, which in this case means it's safe up to 168mph. As with the load rating, you should always have tyres that are rated beyond the maximum speed of your car to maintain a safety margin.

Here are some other maximum speed rating letters:

N – 87mph

P – 93mph

Q – 99mph

R – 106mph

S – 112mph

T – 18mph

U –124mph

H – 130mph

V – 149mph

Z – 150mph+

W – 168mph

Y – 186mph

So, that's how tyre numbers work. Simple really, when you know the code.

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