First Three Digits. (The Tyre Width)
The first three digits indicate the width of your Tyre in millimetres. A tyre marked with 185 will be 185mm across the tread from sidewall to sidewall.
The Fourth and Fifth Digits. (The Aspect Ratio)
The fourth and fifth digits indicate the profile height (aspect ratio) of the tyre sidewall. This number is a percentage of the tyre width. A tyre marked with 60 would indicate that its profile height is 60% of the tyres width.
The Sixth & Seventh Digits. (The Wheel Diameter)
The Sixth & Seventh Digits indicate the size of the wheel rim that the tyre can be fitted to. It is also the diameter of the tyre from bead to bead. So a tyre marked 15 will fit on a 15-inch wheel rim.