I usually rode on Schwalbe Supremes (paved only) or Schwalbe Mondials (mixed surface). I can't remember for sure but the rear was only 37 or 35mm, I "think" 37. I tried a larger 40mm Supreme and it did not fit. The front fork handled the 40mm fine and probably could have done more. My frame was a 56cm if it matters. I enjoyed the Americano but really wished it would have accepted 40-42mm tires in the rear. I am considering a Co-Motion Divide (50mm F&R) but due to an 8 month build time, will probably pass.