Guitar Ted posited the theory that Shwalbe is using UST specs while American Classic closely matches Stans rims which use BSD specs. The result is incompatible tire/wheel combination which neither the tire nor the wheel manufacturer is going to admit because they don't want to limit sales.
See post number six at
https://ridinggravel.forumchitchat.c...6&forum=494958 for a full explanation.
-Timp-