I'm curious if someone can point to a source for that claim.

I recall the rim was reportedly:
(a) optimized for a 23C tire,
... but I was looking for and never saw a claim that:
(b) a 23C tire was optimal for that rim.

Two statements with similar, but not quite the same, meaning.

In my experience testing tires and rims (in CFD and wind tunnel testing), the most aero tire is usually narrower than the rim it's on, and the most aero rim is usually wider than the tire mounted on it.

HED's recent patent puts some numbers on this relationship.

Cheers,
Damon

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)