The tyre sidewall indicates the tyre size. As the tyre sidewalls provide information on its most important parameters, they are subject to the ECE-R 30 regulations in Europe.
A 205/55 R16 tyre has a width of 205 millimetres because this value is given first in the side marking and is expressed in millimetres. The section width of a tyre is the distance between the inner and outer extremes when the tyre is installed on a rim suitable for the tyre size. Tyre sizes change when the difference in width is 10 millimetres. The actual tyre width often differs by a few millimetres from the sidewall marking, as this is due to the specific characteristics of the manufacturer.
The tyre profile is given in the second place in the sidewall marking. For a 205/55 R16 tyre, the profile is 55, i.e. 55% of the width of 205 millimetres, as this value is the percentage of the height of profile (side) to the width of the tyre. In this particular example, the actual profile is therefore 112.75 millimetres. There are low profile tyres with 40, 45, 55 markings and high profile tyres which often no longer have the correct indicators, e.g. the 165R13 has an 80 profile.
The tyre's radial structure means that it uses layers of fibres which run radially across the tyre at right angles. In the side marking, radiality is indicated by the letter R. The 205/55 R16 has a carcass that runs radially from the centre of the wheel. 98% of tyres sold have radial construction.
Number 16 - the last value of the tyre size shows what should be the diameter of the rim on which the tyre is to be installed. Manufacturers indicate it in inches, which is why the name of inch tyres has been adopted.