Question

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Answer 1

What do the various size markings on bicycle tires mean?
Nowadays, bicycle tire sizes are all marked according to ETRTO (European Tire and Rim Technical Organization) European standard.However, older English and French tire size dimensions are still used as well.

The ETRTO size specification 37-622 indicates the width of 37 mm and the tire inner diameter of 622 mm. This dimension is clear and allows for precise classification of the rim size.

Inch Marking (e.g. 28 x 1.40) states the approximate outer diameter (28 inches) and the tire width (1.40 inches). Inch marking such as 28 x 1 5/8 x 1 3/8 (approximate outer diameter x tire height x tire width) is also common.

Inch sizes are not precise and lack accuracy. For example, diameters 559 mm (MTB), 571 mm (Triathlon) and 590 mm (Touring) are all classified as 26 inch. Tires with diameters 622 mm and 635 mm are both classified as 28-inch. Oddly enough, tires with an inner diameter of 630 mm are classified as 27-inch.

These classifications originate from the time of tire brakes. In those days, the exact outer diameter of the tire was defined by the brake. Depending on tire width, various standards for the inner diameter applied.

Inch dimensions are widely used in both MTB sport and English language countries. Therefore, we will continue to use these specifications for all tires. In the future, only inch decimal measurements will be used e.g. 26 x 2.25. Experience tells us that nowadays very few users are familiar with the classical fractional inch dimensions such as, 28 x 1 5/8 x 1 3/8.

When the 29 inch MTB tire size was introduced a few years ago it had the same inner diameter of 622mm, known as 28" in Europe.
French size markings (e.g. 700 x 35C) give the approximate tire outer diameter (700 mm) and width (35mm). The letter at the end indicates the inner diameter of the tire. In this case, C stands for 622 mm.

French size markings are not used for all tire sizes, so, for example, it is not used for MTB sizes.