Gentleman, as promised here are my findings (on my car of course).
with a 255/60/15 BFG Radial T/A tire on a 15x8 rim with 4.25" backspacing my car has about 1/2 on the unrolled fenders and about 3/4" on the leaf springs. Video below.
https://www.youtube.com/watch?v=G4jLZMyEBCw
So if I had 4.5" Backspacing on my rims I'd be looking at 3/4" at the fenders and 1/2 at the leaf springs. A 275/60/15 tire that i wanted is about an inch wider. That would make it pretty close. I don't know if I could say a 275 tire would fit "my car" without rubbing, which I did not want (rubbing issues). It may fit , it may not. But these are my findings if anyone is interested. Thank you all who replied. El 56.