Deciphering Your Car Tyres

We’re always telling you about the manufacturer’s specifications, and how you should use them to determine what tyres and alloys will fit your car best. To make life easier, we’ve compiled a quick guide to what all the numbers and letters mean, which will all be located in the same place on a standard tyre wall. These are essential statistics to know when you’re thinking about buying new wheels and alloys, so it’s worth familiarising yourself with them now.

Aspect ratio
This is simply how the width of the tyre relates to its height. Only the sidewall height is taken into account, so this measurement should be taken from a section and not of how high the tyre stands on the ground.

Aspect ratio is expressed as a percentage, so a tyre with an aspect ratio of 60 will have a height that is equal to 60% of its width. Low profile tyres therefore have lower aspect ratio numbers.

Load index
This number refers to how much weight the tyre can safely carry. You need to look up this number in a tyre load index table, which will tell you how your number converts to a number in kilograms. When buying new tyres, always ensure that you buy those with a load index suitable for your car, otherwise you might not be going anywhere fast!

If you see R on your tyre it stands for ‘radial’. Almost every consumer tyre you’ll come across these days is radial rather than cross-ply construction, but it’s still important to check as you should never mix the two styles.

Rim diameter
This is simply the size of the wheel that the tyre will fit onto. Whatever number you find on your tyre, it will be measured in inches.

Section width
This is the width of the tyre’s tread, measured in millimetres. It’s the other figure used to calculate aspect ratio.

Speed rating
This is, perhaps surprisingly, a letter not a number. It should be a figure between H and Z on the alphabet and corresponds to how fast your tyre can go. S, for example, is limited to 112MPH. Check your letter and then look it up on a tyre speed rating table – this will ensure you stay safe on the road. A quick note on the letter Z. This will appear alongside another speed rating (which will give you a more specific figure) and simply means your car can travel at speeds of more than 150MPH – so you’ll see this a lot on supercar tyres!

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