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2012 Z5 with Zipp Firecrests here (tubular). Vittoria CX 23s are my go to. 25's are so close to the chainstays that the slightest deviation from dry, perfect roads results in grit grinding the frame. The only flaw (other than BB30 which I am philosophically opposed to but has been OK) in an otherwise perfect bike. FWIW, the ride is very smooth, so you won't miss the 25's and the minuscule performance you give up in rolling resistance is made up by a minuscule gain in aero. We all did just fine on 23s for decades.
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