Originally Posted by Sy Reene

Zipp says what is optimized for 25mm?

Per 303S description on sram's site: "The key innovation is in the 303 S Series’ rim design. Its 45mm rim depth is the same as its predecessor, the 302 Disc brake. But its rim width is 2mm wider, at 27mm, and its tire bed is a full 7mm wider, at 23mm. That rim platform is designed to be the fastest with a 28mm tire."

I didn't see any ad copy on the 404 page that has any similar language.

I believe the issue is ETRTO. An inflated 25mm tire on a wide rim would probably be about ideal (105% rule) inflated width from aero perspective on a 27mm width external rim. However, ETRTO is going to hamstring manufacturers by indicating that for a 23c rim, the minimum width tire you're "allowed" to run is a 28mm. For a hookless rim, to bring the internal width down to 21mm minimum (says ETRTO), but keeping the 27mm external, they'd need a thicker bead wall.