I have a new Diverge, and it fits "approximately" 38c with some decent room, and it supposed fits a 42c tire"
"The new frame will comfortably fit up to 700x42mm tires with plenty of room for mud, too."
- from Specialized's website
So when I tried the Sawtooth 42c tires on them, I was quite dismayed to find that they rubbed the non-drive-side chainstay.
I finally got some calipers to measure my tires on various wheelsets and I found some rather surprising results:
38c Specialized Terra Pros + Roval Control carbon SL(i22) = 39.65c (awesome tires, BTW)
42c Specialized Sawtooths + Industry9 245 Trail (i24.5)= 43.60c
38c Specialized Trigger Pros + Roval SLX24 (i20)= 37.2c
I've read that some tires (for example, Kenda Flintridge, Panaracer GravelKing SK, among others) measure notably larger than their actual claimed size.
If there is a running list of actual (measured) gravel tire sizes, I'd like to at least contribute my own findings as well as check other tires. I know there is one for the "plus" sized MTB tires (i.e. the 27.5+, ~3" tires)