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Explaining Tyre Markings

By raccars Published

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All tyres feature markings on the sidewall referring to width, section height, rim diameter, load index and speed index - for example, 215/65 R17 95 H - but an awful lot of car owners don't know how to interpret the digits. Improve your understanding of your car and make sure you replace your old tyres with the correct new ones by teaching yourself what the markings mean.

Tyre width

The first three digits in the series refer to the width of the tyre in millimetres (215 in the example given above) - this number will usually end in a five. The width of your tyre affects both the performance of your car and its fuel economy, because it refers to the tyre's 'contact patch', or basically the area of rubber that makes contact with the road surface. In theory, a larger contact patch results in better grip but higher rolling resistance, which means poorer fuel economy.

Section height

The tyre's height in millimetres as measured by the visible profile. This is the tyre wall from the bead anchoring the rubber to the wheel rim to the upper limit of the tread pattern. The number given - 65 in the example above - is the section height's percentage of the tyre width. Low profile tyres are popular on high performance cars because they offer more grip when taking corners at speed, but are also chosen for aesthetic benefit.

Rim diameter

This is the size of the rim, or the metal wheel that anchors the tyre. Confusingly, these are given in imperial rather than metric measurements, and usually range between 12-22 inches - the example above is R17 or 17 inches. Tyres with a larger diameter result in a larger contact patch and therefore more grip on the road, but again are often preferred for cosmetic reasons rather than performance benefits.

Load index

This is a modern measurement of the maximum weight your tyre can carry, which can be interpreted by finding a tyre load index table online. The figure 95 given above means that tyre can support a 690kg load.

Speed index

Indicated by a letter which can be interpreted online using a speed index table, this shows the maximum speed at which the tyre can travel safely for no more than 10 minutes while carrying its maximum load. Vehicle manufacturers are obliged to give a minimum speed index and fitting tyres below this rating could invalidate your insurance. You can, however, fit tyres with a higher speed rating. The H given above equates to 130mph.

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