Last updated: 2023-08-11
The specification 700x40C in the above example tire is the so-called French size (tire in french). These come in several variants, which are explained below.
Example: 700 x 40C
The French long designation consists of two numbers and one letter. The first number, 700, means that the tire has an outer diameter of 700 mm (i.e. 70 cm).
The second number, 40, indicates that the tire is approximately 40 mm (i.e. 4 cm) wide when inflated. The tire width is measured at the widest point of the tire.
The letter C stands for the approximate tire height, in this case about 39 mm. In total, there are three common heights:
Example: 700C
The French short designation consists of a number and a letter. The first number, 700, means that the tire has an outer diameter of 700 mm (i.e. 70 cm).
The letter C stands for the approximate tire height, in this case about 39 mm. For the other possible letters, see above.
The French designation is somewhat inaccurate, as the rim diameter or tire inner diameter is not explicitly specified. However, it can be estimated indirectly via the tire outer diameter and the tire height. For above examples, the corresponding rim diameter is:
700 mm − 39 mm − 39 mm = 622 mm
Nevertheless, this value for the rim diameter is inaccurate because for the tire height only the three categories A / B / C are used instead of giving an exact value. So when buying a new tire to fit the existing rim, I always additionally pay attention to the ETRTO specification, as this indicates the exact rim size.
This article is part of a series of articles. For the other information on the bicycle tire see:
