I apologize if I am bringing back an old thread but I was bored at work the other day and just thought I'd post my findings. I know that the OP was originally asking about tire clearances with regard to the fork, but after I failed to fit anything larger than a Ritchey 32c SpeedMax Cross tire (on Weinmann DP18's) in the stock IRO fork, I decided that I was going to go a different direction in order to achieve greater tire clearances on my Mark V. So the plan is to swap out the fork for something with more generous clearances. But first I wanted to check what was possible to fit in the rear stays. I was able to fit a 700x35c Bontrager knobby in the rear comfortably, and could probably fit as larger as a 37c or 38c tire. And that is with the wheel only about half-way back in the track-ends. That combined with something like a Steamroller fork and you've got basically a new bike that'll take in up to 700x37c/38c tires.
Check out some of the photos here:
http://www.flickr.com/photos/jonatha...68/5239800429/