Understanding tyre sizes

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Understanding tyre sizes

As standard, every tyre will come with markings indicating information about that specific tyre. The information provided will show tyre size, the tyre manufacturer, speed rating, model and load index.

Understanding the information on the sidewall of a tyre can sometimes be confusing. Below are some tyre markings and their meanings.

165 – This describes the maximum width of the tyre when the car is fully loaded with passengers and luggage in millimetres. In this case, the tyre is 165mm wide.

R – This refers to the construction of the tyre. In this case, R is for radial construction tyre with steel wires. Vintage and classic cars may show as D.

13 – This refers to the size of wheel the tyre can fit. In this case, this tyre can fit a wheel, an alloy or a steel wheel as long as it has a 13” diameter.

79 – This marking indicates the maximum load the tyre can accommodate. In this instance, the vehicle can sustain a weight load of 79KG.

H – Here, H represents the tyre speed rating. In other words, the maximum speed the car can travel without risking the failure of the tyre. In this case, H has a speed rating or 130mph.

What is tyre speed rating?

The tyre speed rating determines the maximum speed you should be driving your car without risk of tyre failure. The higher the speed rating of the tyre, the better the quality of tyre.

What is tyre load index?

The tyre load index is usually in KG and represents the maximum weight the tyre can carry. There is a need for this as all vehicles are different in terms of their load baring, some vehicles will require greater inflation of the tyre and are marked with XL (Extra load) or RF (Reinforced).

Sidewall heights

The marking locations on low profile tyres can be different compared to normal tyres. After the stated width of the tyre, there is a second number that defines the sidewall height of the tyre.

Using the example 165/60 R 13 79 H, 60 indicates the sidewall height. Meaning it is 60% of the tyre’s width.

More often than not, normal and standard tyres will have a sidewall height of 82% of the tyre width.

Can I use a different tyre size?

It is recommended that cars are fitted with tyres that are detailed in the owner’s manual detailing the size, profile and width.

However, it is possible to fit different size tyres with the help of an expert. The offset and inset values will need to be taken into consideration to avoid suspension damage.

When changing a tyre size, you must retain the tyre and wheel diameter to the same original value so that the speedometer and gearing are unaffected.

For every 20mm boost to the width of the tyre, a reduction of 10% in the height of the sidewall is required if the wheel size is unchanged.

It is important to remember that tyres and wheels must fit under the wheel arch without protruding. Police would have to take action with your vehicle if this was found to be the case.

NOTE: If you wish to change the tyres or wheels on your car, your insurer must be notified ahead of the change.