New vs Old ETRTO: Key Differences and Tire Selection Tips

What Is the ETRTO Standard?

The ETRTO (European Tyre and Rim Technical Organisation) standard is a unified specification for tires and rims, established by the European Tyre and Rim Technical Organisation.

ETRTO

When choosing bicycle tires, especially for road bikes, you’ll often encounter two types of size notations:

1. 700 x 25C (French size)
2. 27.5 x 1.50 (Inch size)

However, these size formats can be confusing for users, as it’s hard to know which tire fits which wheel. In addition, the actual tire width can vary depending on the internal rim width, making consistency difficult.

To solve this issue, the ETRTO standard provides a unified sizing system.

While both French and inch sizes are based on the outer diameter of the tire, the ETRTO standard defines tire size based on the diameter of the rim where the tire seats—that is, the outer diameter of the rim or the inner diameter of the tire.

From the tire’s perspective, it’s the inner diameter that matters; from the wheel’s perspective, it’s the rim’s outer diameter. As long as these match, the tire and wheel will be compatible.

In ETRTO sizing:

• 700C is labeled as XX-622
• 650B is labeled as XX-584

For modern sports bicycles, ETRTO sizing has become the norm. For example, Shimano’s official specifications for their new 105 wheelset “WH-RS710” list the recommended tire size as 25-622 to 32-622, using ETRTO rather than French sizing.

Differences Between the Old and New ETRTO Standards

The previous ETRTO standard defined tire dimensions (outer diameter and width) based on a rim internal width of 15mm.

However, in recent years, wider rims have become more common in road bike wheels. The once-standard 15mm internal width has gradually shifted to 17mm, and now 20mm or even 24mm rims are available, even on road bikes.

For example:

• Shimano’s current entry-level wheel model WH-RS100 has a rim width of 17mm, designated as 17-622 (622 x 17C).
• Its predecessor, WH-RS010, had a 15mm rim width, designated as 15-622 (622 x 15C).

WH-RS100-CL-R | Shimano
WH-RS010-CL-R | Shimano

To reflect this shift, the new ETRTO standard has been introduced. It sets the rim internal width at 19mm for tires sized between 25C and 28C.

How Does Changing the Rim Width Affect Tire Width?

Even if the tire size remains the same, the actual installed width can vary depending on the rim’s internal width. A general rule is:

For every 2mm change in rim internal width, the tire width changes by approximately 1mm.

For example, a 25-622 (700x25C) tire designed under the new ETRTO standard (based on a 19mm rim) will appear 1mm narrower when mounted on a 17mm rim. That means the tire will measure 24mm in actual width.

Similarly, on a 15mm rim, the same 25C tire would measure 23mm wide—matching the width of an old ETRTO 23C tire.

Are the Old and New ETRTO Standards Compatible?

Yes—regardless of whether the tire follows the old or new ETRTO standard, as long as the inner diameter of the tire (i.e., the rim seat diameter) matches the wheel, it will be compatible. For example, all 700C road tires use 622mm as the rim diameter, and this remains unchanged in the new standard.

What differs is the width reference. If a tire is labeled as using the new ETRTO standard, the specified width is based on a 19mm internal rim width. When using a different rim width, apply the 2mm-to-1mm adjustment rule mentioned above to estimate the actual installed width.

Points to Keep in Mind When Buying Tires

No Issues if You Pay Attention to the New ETRTO Standard

Tire manufacturers that use the new ETRTO standard typically include the phrase “compliant with the new ETRTO standard” on their official product pages. If this phrase is not included, you can assume the tire follows the old ETRTO standard.

Since the new ETRTO standard was introduced in 2020, most tires developed and designed after that time comply with this standard. However, tires released before 2020 are generally based on the old ETRTO standard.

If you’re simply replacing your old tires with new ones of the same size, you usually don’t need to worry about the new ETRTO standard.

New ETRTO Standard Tires Can Be Installed on 15C Rims

Even with the introduction of the new ETRTO standard, the tire’s inner diameter remains the same. Therefore, you can install tires that follow the new ETRTO standard on older 15mm internal width rims without any issues.

However, when installing a 25C tire designed to meet the new ETRTO standard (which assumes a 19mm internal rim width) on a 15mm internal rim, the rim width will be 4mm smaller and the tire width will be 2mm smaller. This means the tire will actually measure as a 23C size.

As long as you understand the relationship between tire width and rim internal width, you can easily choose the correct tire size.

Pay attention to rim internal width markings when purchasing wheels. These are often labeled as XX C, C XX, or rim internal width XXmm.

Some Manufacturers Have Their Own Tire Width Standards

While the ETRTO standard uses 15mm rims for the old specification and 19mm rims for the new specification, some tire manufacturers have their own unique designs.

For example, Vittoria’s P ZERO ROAD tires are not based on either the old or new ETRTO standards, but are instead designed for a 17C rim.

The P ZERO ROAD is available in three sizes:

1. 700x24C
2. 700x26C
3. 700x28C

For example, when using a 700x24C tire on a 19mm rim:

• Since the P ZERO ROAD is designed for a 17mm rim, when the rim width increases by 2mm, the tire width increases by 1mm, making it equivalent to a 25C tire.
• On a 15mm rim, the rim width decreases by 2mm, making the tire width 1mm smaller, resulting in a 23C tire.

Frequently Asked Questions About ETRTO Standards