225 is the width of the tyre. This is the first value included in the tyre code, expressed in millimetres. The tyre width in different codes increases in steps of 10 millimetres (e.g. 175/65 R14, 185/65 R14, 205/55 R16, etc.) The actual width of tyres often differs slightly from the nominal parameters, with a difference of several millimetres, depending on the tyre manufacturer. The number 225 denotes a width of 225 mm between the most extreme point on the outer side wall and the most extreme point on the inner side wall, when the tyre is mounted on a wheel of the correct width. This dimension is called the cross-sectional width of the tyre.
45 is the aspect ratio. This is the second value in the tyre code. It describes the profile of the tyre, as the percentage ratio of the profile height to the tyre width. In other words, it states what percentage of the width of the tyre is accounted for by the height of its side. For example, in a tyre marked 225/45 R17, the aspect ratio is 45, which means that the height of the side (profile) of the tyre is 45% of its width, 225 mm. In this case the actual height is 101.25 mm. The lower the aspect ratio, the lower the profile of the tyre (values of 50, 45 and 40 denote low-profile tyres). On higher-profile tyres the aspect ratio is often not given; for example, a tyre marked 165R13 has an aspect ratio of 80.
The letter R denotes a tyre with a radial construction. Here the tyre body contains layers of fibres running in a radial direction, across the tyre, at 90 degrees. The R in the designation 225/45 R17 indicates that the tyre has body layers running radially from the theoretical centre of the wheel. Radial tyres are easily the most popular tyres with drivers, accounting for more than 98% of total sales.
17 is the value in inches of the diameter of the wheel rim on which the tyre should be mounted.