I mounted up a set of P-Zero Race's on some older 303s recently, and Pirelli's WAM chart was dead accurate- the Zipps measure 17.25mm internally, and at 95psi, the 28mm P-Zeros measure out to 26.9mm, when their chart said 27mm. I haven't had a chance to put miles on them yet (life...) to see if/how much they plump out with use, but if you have a relatively narrow internal width rim, I bet 26mm will work fine.