The tyre size can be found on its side in the form of a sequence of numbers and a letter.
225 - tyre width This is the first value given in tyre size and is expressed in millimetres. The tyre width in each size increases every 10 millimetre (e.g. 175/65 R14, 185/65 R14, 205/55 R16, etc.). The actual width of tyres often deviates slightly from the nominal parameters and varies by a few millimetres depending on the tyre manufacturer. 225 is the width of 225 mm between the furthest point on the outer sidewall and the furthest point on the internal sidewall, when installed on a rim of an appropriate width. This dimension is referred to as the cross section width of a tyre.
45 - aspect ratio. This is the second value in tyre size. It is an indicator of the tyre profile. Defines the percentage ratio of profile height to tyre width. In other words: what percentage of the width of a tyre is the height of its sidewall (profile). For example, in a 225/45 R17 tyre, its profile is 45, which means that the sidewall height is 45 percent of its 225 mm width. In this particular example, the actual profile is 101.25 mm. The lower the value given in the tyre profile, the lower the tyre (50, 45, 40 – so called low profile tyres). In the case of high profile tyres, the height of profile is often not indicated (for example, 165R13 – which means that the profile is 80).
R - means a tyre of radial construction. In a radial tyre, the tyre body is made up of layers of fibres running radially across the tyre at an angle of 90 degrees. The designation R in a 225/45 R17 tyre means that the tyre has a radial construction in which the carcass layers propagate radially from the theoretical centre of the wheel. Radial tyres are by far the most popular type of tyre, accounting for more than 98% of total sales.
17 - it is a value expressed in inches indicating the diameter of the rim on which the tyre is to be installed.