Car Tyres Teasers - Important Facts You May Not Know

‘We’re on your side.’ This sounds like the tag line in an advertisement but here it refers to the codes on a car tyre’s sidewall. Most of us know the obvious ones like those referring to wheel diameter and tyre section. There are, however, many other pieces of vital information to be found, moulded into the sidewall. Let’s consider some of the more important ones.

Did you know that some car tyres have a marked rating that indicates how well they handle heat build up? If you see a mysterious “A” on a tyre, it has the highest rating. A “C” denotes the lowest.

The traction rating shows how well a tyre can stop you on a wet road. This rating goes from “AA” to “CC”. No prizes for guessing which is the best.

Now for some numbers. Tread wear ratings go from 60 to 600 in increments of 20 and the figure shows how long the tyre is expected to last. In theory, a tyre marked “200″ should last twice as long as one marked “100″. This is, however, a relative rating. The actual tyre life will depend on many factors, including your driving style, the road surfaces encountered, correct wheel alignment and the right tyre pressure. Think ‘benchmark.’

The speed rating a tyre carries is extremely important. Using over-specified tyres is a (very expensive) upside here; tyres for high-speed use can be used at lower speeds. On the other hand, if the speed rating is too low and you have an accident, your insurance company may take a dim view of the tyres your car was wearing at the time. Speed ratings run from “L” (75 mph) to “Z” (150 + mph). Currently, H-rated tyres (130 mph) hold the largest percentage of new tyre sales.

So much for speed, what about load? Yes, car tyres tell you about the load they are capable of carrying, They have a marked load index (LI), which is valid for sub-130 mph speeds. Load indices range from 50 (190 Kg) to 169 (5,800 Kg) but the calculations are a touch complex. Let’s say you have a car that weighs 2 tonnes, or 2,000Kg. Divide this by four to get the weight on each wheel; this equals 500Kg. So far so good, but, do you know that all four wheels carry 500Kg or is one end of the car heavier than the other? In practice, it’s usual to add a percentage to cover such factors.

In our example, then, the car needs tyres with a load index of 84. We can add 20 percent, giving a weight bearing capacity of 600Kg. Therefore tyres with a load index of 90 will do very nicely. In reality, a car tyre’s load rating will exceed its actual needs by a long way. However, when speeds reach 130 mph or more, the load index calculations change dramatically. For reasons far to technical to explain here, the theoretical load rating decreases as road speed rises. In the event you run a very, very fast and heavy car where the speed cameras don’t work, there are experts in such matters. Find one!

Lastly, you should never use a tyre more than 6 years old. The date of manufacture is part of the DOT code marked on one sidewall. For example, a tyre marked “2709″ was made in the 27th week of 2009.

Merityre are a leading UK independent supplier of car tyres. Why not visit their website at www.merityre.co.uk and see where you can buy your next set of tyres.