Despite being black and round and all of them looking very similar, tyres can be subject to many variations on the theme. For a piece of quick revision, most of us are familiar with the sizing system used on modern radial tyres. A quick review follows with reference to item B in Figure 1: 185/65R13 is the nominal dimension of the tyre. Breaking this code down: 185 is the nominal width of the tyre, ie over the sidewalls when the tyre is inflated to its recommended pressure. This should not be confused with the width of the tread face, which normally is not as wide. In this example it is 185mm. /65 is the height of the tyre sidewall (section height) expressed as a percentage (%) of the tyre width. In this example 65% of 185mm, this is 120mm. R indicates that the tyre is of RADIAL construction - most vehicles in the pare are using radial tyres. 13 is the diameter of the rim which the tyre is designed to fit onto. In this case it is 13 inches. Some OEMs have used metric rims in the past; most noteworthy were Austin Morris/Austin Rover with the Metro 85. This came equipped with 160/65R315TD as standard factory fit.
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