Cross post from the Road Forum ( my guess is the Road poster have more experience with Zipps, You guys obviously have tandem experience.)
First, I understand this is not the best answer from an aerodynamic point of view. However these wheels are 808 rims, (520's) laced to tandem hubs, and are going to be used on a tandem. For crits, I want to use 25mm tires (Continental 4000's) for the flat protection, and cornering grip.
However Zipp says to only use 18-23mm tires with the wheels. My first thought was to check Sheldon's website. From that it would appear even the narrowest rims will work with 25mm tires.
Unfortunately the interior dimension of the 520 Zipp rim is not readily available. The website lists the "aero rim width" at 24mm, but that does not refer to the interior rim width.
The tires mount on the rims with no issues, and are very tight on the rims.
So HTFU,and go ride them, or listen to Zipp?