Tyre size is on the tyre sidewall. Marking is carried out in accordance with the regulations of ECE-R 30, the law in force in Europe. These rules cover to the greatest extent the designations from which key data on product parameters can be read.
185 - tyre width. The very first value in tyre size, expressed in millimetres. Tyre width varies by 10 mm depending on the size, so examples of differences in size may be as follows: 175/65 R14, 185/65 R14 or 205/55 R16. The actual width of a tyre usually differs from the information on the sidewall inscription, depending mainly on the manufacturer's preferences. Number 185 denotes the width of 185 mm of the distance between the furthest point of the outer sidewall and the corresponding point in the inner sidewall when the tyre is correctly installed on a rim. This length is referred to as the cross section width of a tyre.
65 - aspect ratio. This is the value after the width of a tyre, which serves as an indicator of the profile. It informs the user how much of the tyre width value is the profile of a model. In other words, what proportion of the width of a tyre corresponds to the height of its sidewall in percent. For example, for tyre size 185/65 R15, we read a profile with a value of 65, so the sidewall height of the tyre is 65% of the 185 mm product width. As a result, we find out that the tyre profile (aspect ratio) is 120.25 mm. A low level of profile value is characteristic of values 50, 45 or 40, such models are referred to as low profile tyres. In the case of high profile tyres, the height of the profile is unlikely to be indicated, e.g. a 165R13 tyre has a profile of 80.
R - indicates that a given tyre is of radial construction. The body of a radial tyre consists of layers of fibres running radially, across the tyre and at right angles. The R symbol indicates that tyre size 185/65 R15 is made up of carcass layers spreading radially from the centre of the wheel. Radial tyres are the most popular tyre for drivers, accounting for 98% of all products sold.
15 - thanks to this value we can find out how much is the diameter of the rim on which a given tyre should be fitted. Manufacturers present it in inches.