A = Width of tyre (in millimetres)
In this example it shows that the tyre is 185mm wide.

B = Tyre profile as a percentage of width (A).
In this example the ratio is 60% of 185mm. This works out as 111mm.

C = Diameter of inner rim (in inches).
In this example it shows that the rim is 15" in diameter.

From this you can also work out your rolling circumference.

e.g. (2 × B) + C = diameter = d

therefore (2 × 111) + (15" × 25.4) = 222 + 381 = 603mm

circumference = π × d = 3.142 × 603 = 1895mm