As far as I know, each type or size or whatever of tyres has its own set of numbers which presumably mean something. In the instance of my last car these numbers were 195/60/R15.
What do each of these numbers represent?
The first number is the width of the tire. In this instance the tire is 195mm in width.
The second number is the percentage of the first number that makes up the height of the side wall of the tire. In this instance it is 60% of 195 which is 117mm.
The final number is the diameter of the wheel that the tire fits on - in this instance the diameter is 15 inches.