In Europe tire sizes are indicated in the following ways:
German classical approach
The tire sizes in inches with two numbers, for example, 28 × 1¾, wherein the first number is the outside diameter of the inflated tire indicating inches in nominal and the second number is the height from the wire to the top as well as the width.
The size in inches by three numbers, such as 28 × × 1⅝ 1⅜. This tire has the same rim size as 28 × 1⅝, but the height and width are 1⅜ “and the outer diameter is not 28”. It is not true that the third number indicates the width.
In the French system, for example 700-23C or 700-38C, is the first number the outer diameter of the tire in millimeters. The second number stands for the width of the tire in millimeters.
The various old systems are gradually replaced by a European ETRTO standard.An example is 47-622, 40-622 (city bike) and 23-622 (racing bike).
Hereby is the first number the width in millimeters of the inflated tire and the second number indicates the rim size (the diameter of the rim), measured where the inner tube rest on the rim.
This corresponds to the diameter of the bead wire of the tire and constitutes an important difference with the other systems, where the outer diameter of the tire is measured.
The European ETRTO standard is used in industry since the 80s of the 20th century. Old designations usually can not be converted to metric sizes.
ETRTO standard gives correct sizes, and can therefore measure again if it is correct.
That is not possible with inch sizes, even as illustrated by the fact that a 27-inch tire needs a larger wheel than most 28-inch tires.