I have a 2018 Canyon Ultimate CF SLX Disc. When I recently acquired a new wheelset, I wanted to use my old Specialized CL 32 for gravel, but I couldn’t tell how wide I could go. The official Canyon spec is 30mm, not much for gravel.
It turns out the answer is 35MM. I used a 700x33c (33mm) Vittoria Terreno Dry, which expands to 35MM at 40 PSI. There’s still enough clearance that I’m not afraid of dirt and a little mud. For reference, the CL32 wheelset is 21mm internal, 27mm external.
If you’re curious if 35MM is wide enough for gravel, the answer for bike-handling weenies like me is probably no. BUT for Mathieu Van Der Poel, “the only obvious change from the usual race-spec is the low-profile Dura-Ace C36 carbon tubeless wheels shod with what appear to be Vittoria Terreno Dry tyres in an assumed 35 mm width (to match the available room in the Ultimate frame).” (MATHIEU VAN DER POEL IS RACING A ROAD BIKE AT GRAVEL WORLDS)