Why are tires sometimes narrower than their listed size?

In order to ensure that tires have sufficient frame clearance, tire manufacturers generally prefer to keep production closer to the lower end of the permitted tolerance (+/- 2 mm).
Once mounted, inflation pressure plays a major role in the tires measured width. With maximum inflation pressure the tire becomes wider than with low pressure. This continues until the tire is fully stretched out. Such a break-in period can require 40-50km of riding before the tire has stretched out to it's final width.

It's Important to note that a tires measured width is directly affected by the internal rim width of the wheel it is mounted on.
A larger internal rim width will yield a flatter cross-sectional tire profile.
Conversely, a smaller internal rim width results in a tire profile that is more rounded.

See in the below tire schematics how a 40mm wide tire can be as narrow as 39mm on a 19mm internal width rim, but as wide as 45mm on a 25mm internal width rim:

When upgrading a tire, one should consider the internal rim width of their wheel as this will dictate if the new tire will be smaller, or larger then the width listed on the sidewall.

In such instances, and since the interior of a tire is a fixed volume, the corresponding width of these tires may be narrower in response to the taller profile.
Additionally, this ensures tire fitment across all maximum frame clearance dimensions as listed by frame and fork manufactures.

Updated on: 08/21/2026