ETRTO standards for 650B tires and their inch counterparts

Types of tire size standards and how to read them

“650B” is a French size standard

The basic premise is that 650B, which is used for 700C road bikes and some gravel bikes and MTBs, is a French standard.

The French standard uses the notation “tire outside diameter x tire width”. All numbers are in mm.

For example, 700-38C means “700mm OD with 38m width C tire width”. The alphabetical part is from A to D. The closer to A, the thicker the tire is.

Tire inch specifications

On the other hand, for MTB and other types of bicycles, the inch standard is used as the major notation. The inch standard uses the notation "tire outer diameter x tire width x tire height. All numbers are in inches.

For example, a tire that is 26 x 1-1/8 x 1-1/2" would be 1.125" wide and 1.5" tall with a 26" outside diameter.

Recently, tire heights are increasingly not indicated.

ETRTO is commonized by tire inner diameter.

The French and inch standards described above were based on the outer diameter of the tire, but the outer diameter of the tire varies depending on the thickness of the tire and the assumed rim width of the wheel.

ETRTO is a common standard that uses the tire width minus the inside diameter of the tire to indicate the size of the tire. The French size 700-38C mentioned above would be “38-622” in ETRTO.

Since ETRTO is based on the tire inner diameter, it does not matter if the outer diameter is larger or smaller depending on the tire size, as long as the wheel rim meets this standard.

By using the tire inner diameter as the standard, the problems of “tires being difficult to fit” and “tubeless tires having a large performance difference depending on the compatibility” are being solved.

For example, Mavic, which has been introducing tubeless for more than 20 years, sold tubeless wheels with Mavic tires until around 2019, when tire and wheel manufacturers began to match the new ETRTO standard established around 2019. Mavic has determined that there are few compatibility issues with tires other than its own, and has stopped selling tire sets for 2020 and later models.

650B gravel tires whose notation is often ambiguous

As we have seen, there are three tire size designations: “French,” “inch,” and “ETRTO.” While most road tires are described in French and MTB tires are described in inches, gravel tires, which fall in between the three designations, are often described in different ways. This is true for road users as well as MTB users. This is probably because both road users and MTB users may use them.

The same gravel tire may be listed as 650 x 40B on one site and 27.5 x 1.50 on another.

This kind of confusion can lead to questions such as, "Which tire is which size? or "Can I install this tire on my wheel? or "Can I mount this tire on my wheel?

French, Inch, and ETRTO compatibility chart for 650B sizes

We have created a chart of the correspondence between the three standards. These are just general sizes, so the sizes stipulated by each manufacturer may differ, but I think the basic sizes are correct.

ETRTO	French	Inched
30-584	650 x 30B	27.5 x 1.20
32-584	650 x 32B	27.5 x 1.25
33-584	650 x 33B	27.5 x 1.30
35-584	650 x 35B	27.5 x 1.40
37-584	650 x 37B	27.5 x 1.40
38-584	650 x 38B	27.5 x 1.50
40-584	650 x 40B	27.5 x 1.50
42-584	650 x 42B	27.5 x 1.65
43-584	650 x 43B	27.5 x 1.75
45-584	650 x 45B	27.5 x 1.75
47-584	650 x 47B	27.5 x 1.85
48-584	650 x 48B	27.5 x 1.90
50-584	650 x 50B	27.5 x 2.00
54-584	650 x 54B	27.5 x 2.10
55-584	650 x 55B	27.5 x 2.15
57-584	650 x 57B	27.5 x 2.25
70-584	650 x 70B	27.5 x 2.80

In some cases, such as on mail-order websites, only either French or inch sizes are indicated due to the character limit of the product name.

However, most tire manufacturers these days list both ETRTOs, so it is basically best to check the ETRTO on the tire manufacturer’s website or catalog to confirm the corresponding French and inch sizes.