Let's see your 20's

[DLW]

You have to get proper back space wheels. You can use spacers but not my preferred method unless they are the billet bolt on kind.

I have 20x10's on rear (a 10" rim is actually 11" wide) with a 275-45 on it. I have about 7/8" gap on the inside fender well and about 1/4" gap on outside. The outside fender lip can be trimmed down quite a bit as well. for more room. My wheel comes out to have about a 4.25" BS. I am dropped all the way to where the differential is about an 1/8" from hitting the bed when aired out and I have no clearance issues.