If you see a road tyre with the measurements in the format below:
Then the following process allows you to determine the dimensions of the tyre:
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.
D = the load rating, e.g the maximum load that can be placed on the tyre.
E = Tyre speed rating.
Read the full article, including information on Loads per tyre, tyre speed rating, and cycle tyre measurements