This is the European tire and rim standard. ETRTO stands for European Tire and Rim Technical Organization.
The size is specified via a two-digit and a three-digit number block. The blocks are separated by a hyphen. The first number block stands for the tire width in millimeters and the second number block for the inner diameter of the tire in millimeters.
The tire size is specified using a three-digit number block, followed by an x (multiplication sign) and a two-digit number block, which is combined with a letter.
The outside diameter of the tire is given in millimeters on the three-digit number block. The special feature of the French abbreviation is the letter. This indicates the tire height.
A = tire height of about 30 mm
B = tire height of 33 mm
C = tire height of approx. 39 mm
Size in inches
The indication in inches has the disadvantage that they only roughly reflect the tire diameter. The customs information is not precise and unambiguous. There are two ways in which sizing is constructed in inches:
1) The size is specified using a two-digit number block and a decimal number with two decimal places. The numeric block is separated from the decimal number by an x (multiplication sign).
2) The size is also given via a two-digit number block, but the tire width is not given as a decimal number, but as a fraction.