21mm is fastest aerodynamically. If you use the search function this has been discussed ad-nauseam

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Heath Dotson
HD Coaching:Website |Twitter: 140 Characters or Less|Facebook:Follow us on Facebook

I know this has been discussed extensively but based on my experience with the 303 clinchers, I think 21mm is as narrow as one would want to run. I was using Vittoria cx evo open corsa II's in 700x25c which meaured ~27.2mm wide when inflated to 100 psi where as the same tire in 700x23c measures ~24.2mm wide inflated to 100 psi and appears approximately the same width as the top of the brake track.

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I used to have the zipp 900 disc and 808 both tubular (non firecrest 2009/2010) and raced with Continental Competition tubular 22mm on the 900 disc and 19mm on the front 808..... I still have a pair in excellent conditions but want to know for when I have to purchase new ones......

In run the same 808/Super9 tubies with a 21mm front, 23mm rear. Rapp suggested that setup and it's worked great for me.

Great combo!! which tires are you using?

According to Zipp, the Firecrest rims can run a tire as narrow as 18cc.

I know they can but Im trying to figure out the best performance tire combo!

Zipp Tangentes. Just be careful to look at the tape before you start gluing. Some have an bulging overlap by the valve stem that creates a hop. If you get one like that make sure to exchange it.