As our good friend Rudy / zonatandem might offer....

We've put 50k miles on 34t timing rings on five different tandems over the past decade two-gether to no ill effect, on road and off-road and I'll be darned if there's any detectable loss of efficiency vs. the 42t ones that came on our '95/'96 Santana Arriva.

Why 34t? Because that's the way daVinci sells 'em. Actually, the only real decision we have to make when it comes to buying timing rings is, "what color would you like: ti grey or black anodized?

Bottom Line: So long as both timing rings have the same tooth count, timing ring size is only an issue if you have a chain stay clearance issue or play around off-road on technical single track where tall timing rings are apt to lead to more broken sync chains than shorter rings...