What do all the letters and numbers on tyre walls mean?

The speed rating indicates the maximum speed the tyre can safely sustain for extended periods. 

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The size of most tyres is marked on the sidewall with a whole lot of numbers and letters. Most of them don’t seem to tally with either centimetres or inches. Why so mysterious? “Mystified”.

The markings on most tyres these days are internationally similar, but they still carry vestiges of more than 100 years of muddled imperial/metric history and detailed differences, additions and omissions between different markets and makes.

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Note: The results are not exact but very close to the actual.