Correcting article on tyre sizes from Jan 22

January 25, 2010 at 12:09 am 1 comment

No wonder the GP (general public) gets confused. Another misinformed article was published in “Drive” on Jan 22.

Their motoring correspondent, Jonathan Hawley, should stick to something he knows something about, or else research it more thoroughly.

For his information, and for the thousands of misinformed readers, the following are the facts:

  • First, the “R” in the tyre size branding indicates that the tyre is of radial construction, not a speed rating.  There are still tyres around of bias ply construction, and they cannot be mixed on the same axle as radials.
  • Second, the last sequence of numbers (eg 93W) quoted in the misinformed article, is a “Service Index”, which is a combination of the codes for the load that a tyre can carry at a defined top speed.

A tyre can carry more load if you slow it down , which is why those giant low loaders that carry gigantic loads on myriads of wheels, always inching along, holding up the traffic, even at 3 o’clock in the morning. So if you specify a load carrying capacity, you must also specify the speed at which you can carry the load.

  • Third, the first group of figures in the size code (225 in the example quoted) is NOT the tread width. It is the width of the tyre measured at its widest point, whilst inflated on a specified wheel rim. The rim width has to be specified (called surprise surprise, the “measuring rim”) because as the rim width increases, so does the tyre width.

The tread width cannot be measured accurately, because the edge of the tread may have decorations such as flutes or scallops, lettering, buttresses or may be contoured, so that a measurement that can be directly compared to tyres of other designs is not possible.

So if you REALLY want to know what the codings on a sidewall of a tyre mean, then consult our “All About Tyres” section of , and be accurately informed.

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1 Comment Add your own

  • 1. BMW Repair Melrose MA  |  April 8, 2010 at 12:05 am

    Don’t you just love all these Authority Sites online today (and their article writers)? Great job Tyre Guys. Thanks for the wisdom of real professionals.


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