On the sidewall of a tyre, we usually find important information about the intended use of the tyre, its dimensions, load capacity and resistance to the increase in temperature characteristic of fast driving. In Europe, tyre markings are arranged in accordance with the regulations of ECE-R 30. The most important element of the markings described in these rules is size.
175 - tyre width. Information on tyre size expressed in millimetres. The larger the tyre size, the higher the marking value which increases by 10mm in succession. Examples of tyre size designations are 175/65 R14, 175/55 R16 or 185/65 R14. In practice, the width of tyres deviates slightly from the values indicated on the markings and varies by a few millimetres depending on the manufacturer of a given model. The number 175 means that the tyre width is 175mm. This value indicates the distance between the outermost sidewall point on the outer wall and the outermost point on the inner sidewall after the tyre has been fitted on a rim. This dimension is referred to as the cross section width of a tyre.
65 - aspect ratio. The next number present in the tyre size indicates its aspect ratio. This value indicates the percentage ratio of the profile height to its width. In other words, this figure tells us how much of the width of a tyre is the height of its profile. Using the example, with size 175/65 R14, the tyre profile is 65, so the sidewall height is nothing more than 65% of the model width, i.e. 175 mm. Based on this example, we can find out that the actual aspect ratio is 113.75 mm. Tyres with a lower profile are called low profile tyres, usually marked with a value of 50, 45 or 40. In the case of high profile tyres, the height of profile in the tyre size is sometimes omitted, e.g. in 165R13 the tyre profile is 80.
R - refers to tyres of radial construction. In radial tyres, the body is designed on the basis of layers of fibres running radially, i.e. across the tyre and at an angle of 90 degrees. In other words, if a tyre has the R symbol in marking 175/65 R14, we find out that the model has a radial structure and has layers of carcass spreading from the centre of the wheel radially. Radial tyres account for 98% of all models sold.
The diameter of the rim on which a given tyre is to be installed is the last number in the side marking - 175/65 R14. In this case, 14 indicates the wheel diameter. This number is given in inches.