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  Tyre knowledge

From size to speed rating, repairs to run flats get to grips with everything you need to know about tyres!

Tyres are the most important safety feature on your car, carrying its weight and dealing with the forces of accelerating, braking and cornering. The diagram below illustrates the information that’s moulded onto the side of every tyre. You’ll need this when getting a quote for new tyres.

Tyre speed rating, load index and size

1. Replacing Tyres

Tyres should always be replaced in pairs i.e. a front set or a rear set to avoid any handling or balance problems under braking.

2. Choosing tyres

Always choose the best tyres you can afford. Budget tyres can provide similar grip to premium tyres in the dry, but there is often a much bigger difference in their wet weather performance. It’s especially important to buy tyres that perform well in the wet as it rains an average of 155 days a year in the UK! (See Winter tyre facts)

3. Tyre size

It is a legal requirement that both front tyres are the same size and both rear tyres are the same size. However, the front set can be a different size from the rear set. A wheel will only accept certain tyre sizes and it’s best to stick with the manufacturer’s recommendation. If you do change the size of the wheels, the accuracy of your speedo will be affected (larger wheels will underestimate your speed). Lower profile tyres tend to make the ride more bumpy and increase noise inside the car.

4. Speed rating & load index

The load index and speed rating of a tyre is shown by a letter and a number, for instance 79T. The number refers to the maximum load the tyre can carry and the letter refers to the maximum speed at which it can carry that load.

For example, from the tables below; 79T = 437kg at 118 miles per hour.

From the speed rating table you can see they are all rated in excess of the UK speed limit. However, the speed rating gives an indication of the tyres all round performance and its ability to cope with braking, cornering and accelerating.

Don’t be tempted to go for a lower speed rating than recommended by the car’s manufacturer. You could end up compromising the car’s handling ability and your safety as a result. The Load Index table doesn’t list all possible values but covers most normal road cars. Values are per tyre.

Tyre speed rating

Tyre load index chart

5. Balance

Tyres are never completely evenly weighted and tend to have heavy spots. When the tyres are rotating, these heavy spots can cause vibration through the steering wheel, making the car uncomfortable to drive and causing unnecessary wear to the tyres, suspension and steering. A wheel can be balanced by using a machine to find the heavy spot and a weight added to counteract it. Wheels should always be balanced when the tyres are replaced, but will also need balancing during their lifetime as the tyres wear.

Wheel balance weight

6. Tracking

Tracking is the direction the wheels face in relation to each other and should run parallel. Poor tracking can cause a car to pull to one side and is a common cause of premature tyre wear, often visible as one side of the tread being more worn than the other. Driving over pot holes or hitting a wheel on the kerb can cause the tracking to become misaligned. To prolong the life of your tyres, suspension and steering, have your tracking checked every 10,000 miles.

Wheel tracking

7. Directional Tyres

High performance tyres sometimes have tread patterns which are only designed to rotate in one direction. This generally improves handling and grip but when fitted the wrong way round, aren’t able to clear water effectively and may cause the car to become unstable. If you get a puncture and the spare tyre is directional, you may have no choice but to put it on the wrong side of the car. If you find yourself in this situation, drive carefully and at a reduced speed.

Directional tyre

8. Repairs

In some cases a punctured tyre can be repaired rather than replaced but only if the puncture is within the tread area and if it hasn’t been driven too far when flat. Damage to the side wall creates a serious weakness in the tyre and should always be replaced.

9. Space Savers

Many modern cars are now provided with ‘space saver’ spare wheels to save weight and boot space and are thinner than the normal wheels of the car. A space saver should only be used for a limited distance, usually around 50 miles and at a maximum speed of 50mph.

10. Mirrors

The mirrors need to be in place and secure, i.e. not held to the car with sticky tape and string. The glass shouldn’t be cracked or smashed.

11. Run flat tyres

A run flat tyre is designed to let you carry on driving with a puncture without the need to change the wheel. As with space savers, run flat tyres should only be used for a limited distance and at a reduced speed.

Also see: How to check tyre tread depth | Facts about winter tyres | How to change a wheel



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