Tyre size can be read on it's sidewall as a combination of digits.
225 - tyre's width.This is the firs value given inevery size in milimetres. Tyre's width in each size raises by 10 mms (e.g.. 175/65 R14, 185/65 R14, 205/55 R16, itd.) True width value is different and slightly different from the given value about couple of milimetres, and measured between the longest points on both: inner and outer sidewall; fitted on a propper wheel. The width is different with every manufacturer.
45 - tyre's profile - hight. This is the second value given; it means the hight of the tyre ( profile ). The value is calculated by percentage value taken from the width. Another words: percentage amount from the width is tyre's height ( profile ).225/45 R17 In this particular case the profile is 101,25mm The lower value the lower the tyre is ( 40, 45, 50 - so caled low-profile tyres ). In case of a full-profile tyre, its height is not given: e.g. 165R13 is a tyre which height is 80%.
R - means Radial tyre. In this design, the cord plies are arranged at 90 degrees to the direction of travel. The "R" in tyre of 225/45 R17 means that the tyre has a radial construction in which the cord piles radiate form the theoretical middle of the wheel. Radial tyres are definitelly the most popular kind among all sold.
17 - is a value given in inches ( " ) and specifying on what wheel the tyre is to be placed.