I am inclined to think it is the fork rather than the tyre which is causing the lack of clearance.
Just did some measuring.
28mm Ultremo ZX on 19mm Mavic rim at 100psi was 29mm wide.
28mm Ultremo ZX on 23mm Hed Belgium rim at 100psi was 30mm wide and 0.8mm lower in profile.
Last edited by Dean V; 06-12-13 at 07:16 PM.