I went and dug out the bike (been on the treadmill a lot instead, bleh). Turns out my GK SK 38's measure 40.
These are 19mm inner width I think.
The fenders measure a touch wider, and it looks like a ton of room between them and the tire.
These SKs are funny, they sound like an aircraft on pavement. Felt stiffer then my riddlers but were quicker and smoother. Not awful on pavement but I'm going best of both worlds with two wheelsets so I can gain even two mph; over ten hours that's like an extra hour!. These SKs are great for everyday rides. Have yet to do a gravel century on these like the riddler's, I really made use of the Riddler's larger side lugs a few times encountering soft spots.