On each tyre's sidewall there is this tyre size
Due to the fact that the tyres are provided with the most important parameters on the sidewall, they are the subject of ECE-R 30.
Tyre205/55 R16 has 205 of milimetres width which is at the first place in this numerics. It is measured between the longest outer and inner point on the sidewals. Tyre sizes differ by 10 milimeters for every next size. True width is different from the markings on the sidewall due to the manufacturer's specification.
Profile is given at a second place in the mentioned markings. Tyre 205/55 R16 has a profile 55, which is 55% of 205 milimetres of width. In this particular case the tyre is 112,75 milimetres high.There are low profile tyres of 40, 45 and 55 and full profile, which do not have the proper profile markings as the profile is 80.
Tyre's radial konstruction means that the pile cords ar at 90 degrees to the bead. In the tyre description the radial build of a tyre is indicated by the R. In the tyre205/55 R16 carcas is radiating from the centre of the wheel. 98% of tyres sold are radial tyres.
The number 16 - indicates the size of the wheel on which the tyre is to be placed. Manufacturers give the size in inches.