ZR, SR and R Tyres, What's the difference?

During the research on the blog about tyre sizes (Imperial VS Metric and Cross-Ply VS Radial), I stumbled across a Classic MINI sporting 165/70SR10's. Now, this size confused me somewhat as normally the Radial number is after the R (R10 for instance) or the more modern variation being ZR17 (generally found on tyres above 17 inches). But, what's this other letter? Why is it SR10? Much like the ZR17, the letter prior to R stands for the speed rating of the tyre. Z means that the tyre can go above 150MPH. Elsewhere on the tyre will be the maximum speed rating of the tyre, it could be a H, V, W or Y. See the blog here on speed ratings!

Knowing this, seeing SR threw me off a little. But, after further research, it looks as though 'back in the day' tyre manufacturers were also inserting the speed rating before the Radial number. In this instance, the maximum speed for an S rated tyre is 112MPH.

R for Radial is the configuration of the in the construction of the tyre. Stronger tyres with higher load rating typically have more ply layers. A van for example would typically have 8ply whereas a car tyre would usually have 4 ply.

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