copied from a website so people can understand how the xxx/xx Rxx is worked out

Size Coding Revealed
A tyre is referenced as"185/60R/15H", here is an explanation.
A/B/C



A = Width of Tyre (mm)
In this example it shows that the tyre is 185mm wide.
B = Profile as a % of width.
(i.e. profile is 60% of width)
In this example the ratio is 60%. This works out as 111mm.
C = Diameter of inner rim.
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 x B) + C = diameter = d
therefore (2 x 111 = 222) + (15" x 25.4 = 381) = 603mm
circumference = pi x d = 3.142 x 603 = 1895mm