Hi. I have a Micargi streatch beach cruiser. It originally had 3" x 24" tire on a slim 2" rim. I mounted a Nuvinci n380, on a 57mm x 24" rim, and spread the rear chain & seat stays using a scissor jack (spreading rear on a different thread).
Originally was a 68mm x 2-1/4 (or 2-3/16)...