How is a tyre size defined?

By Julia Freeman

If you are confused by the arcane code letters on the sidewalls of your car or motorcycle tyres you are by no means alone. Most of them are there to denote the tyre size and some other key parameters that determine whether they will be suitable for your vehicle. In particular, those other parameters include the load bearing strength of the tyre and the maximum speed at which it can safely be driven. Those parameters can be greater, but never less, than what your vehicle is designed for so always check your vehicle manual for the minimum values you need. If you are ever in doubt, ask our tyre professionals at your local Protyre garage.

tyre with sizing's on

You should always fit the recommended tyre size. Although some vehicles can accommodate slightly different tyres it is never recommended: it can throw off vehicle electronics like traction control and may invalidate your vehicle insurance. Swapping in a wheel set with bigger rims and less rubber but the same overall diameter will fit, but your performance will suffer. You will most likely have a rougher ride, slower acceleration and higher fuel consumption.

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Reading your tyre parameters

Most manufacturers now use a similar format to display tyre size on their tyres, but they do space them out differently and occasionally add additional characters for various purposes. It is usually easy to identify the most important letters and numbers. Let's look at an example:
 
205/65 R16 95V 
The first part, 205, tells us the width of the tyre in millimetres. The next number, 65, is called its "profile" and is the ratio of the rubber's depth (or height) to its width. The letter "R" tells you this tyre has radial construction (run flat, RF, and bias, B, are common alternatives). The third number, 16, is the diameter of the metal rim in inches. The next number is usually the tyre's load index (our example has the code 95). Finally, it is followed by an alphabetical letter, V, which denotes the safe maximum speed.
 
The last two parameters are both codes, so you need to consult a table if you want to translate them into real values. A tyre with load index "95" can support up to 690kg and the letter "V" denotes a tyre that is structurally safe up to 149mph. The vehicle should never be capable of reaching that speed nor should it ever reach 690x4 kg fully laden. Other things you may see include a manufacture date or an "M/C" to denote a motorcycle tyre.

The local garage you can trust

Always get the tyre size recommended by your manufacturer, but feel free to choose the tyre brand. Big tyre makers like Pirelli, Bridgestone and Continental offer a huge range of tyres optimised for economy, fuel efficiency, fast handling, durability, cold weather and so on. For example, whatever your tyre size, the Bridgestone Turanza is a great road tyre while the Bridgestone Blizzak is great for snow and mud. The Protyre website offers great tyre deals at competitive prices and is the ideal place to compare tyres. As well as tyre sales we are happy to provide a free tyre safety check.

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About the author

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By Julia Freeman
Julia is Head of Retail Marketing for Protyre and loves engaging with customers and the business as a whole to make sure Protyre is more than just a local garage.
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