Tyre sizes are expressed as such:
111/22R33
The first three digits are the tyre width in millimetres. Using my car as an example, I have 205 size tyres, so they are 205mm across.
The second two digits are the sidewall height as a percentage of the first three digits. I have 55 size tyres, so they are 205 * 0.55 mm high, or 112.75mm.
The R simply says that the tyres are of radial construction. It's very uncommon to find anything else.
The last two digits are the tyre's inner diameter in inches. I have 15 size tyres, so 15 inches across in the middle.
With the given tyre size, 320/30, the tyres are 320mm across and 320 * 0.3 on the sidewall, or 96mm.
My other car has 185/60R14 tyres. I want to replace them, as they suck, with a different tyre, which unfortunately doesn't come in that size - the only 14 inch wheels the manufacturer makes tyres to fit are 195 size. In order to maintain the same rolling radius (so that the total diameter of the tyre is the same, and the speedometer isn't thrown out), I have to adjust the sidewall accordingly - 185/60 is a 111mm sidewall, so I need a tyre with approximately that sidewall size on a 195 tyre. 111/195 is 56.9%, so a 195/55R14 tyre will be an adequate replacement for the 185/60R14s.
