The tyres in size 225/45 R18 are installed in passenger cars of a sports character. The number 225 in the first position is a tyre's tread width in millimetres. The next position is reserved for the tyre's profile height which in this case amounts 45% out of 225 millimetres of the tyre's tread width. The "R" in the third position denotes the radial construction of the tyre. The last number in the size is the diameter of the wheel's rim which can be installed with the tyre in size 225/45 R18 in this case it is 18 inches.) You're welcome to do the shopping!