Demon rear tire/wheel combo question??

What rear axle are you running? BBP or SBP? What springs in the back?

Stock wheel wells and opening by themselves are pretty limiting. The 3" relocation only buys you about an extra 1/2" of space if the stock wheel tub stays in place. Unless you raise the ride height significantly in the back, you won't get much more than a 275/60/15 in there.

I have 295/35/18's on my Duster (same wheel tub size as a Demon). To do that I did a 1/2" spring offset and cut about a 1/2" off my quarter lips. The 295/35/18's have a section width of ~11.9", but I'm still at the limits. Before I added my rear sway bar I was getting some rubbing on both the inner wheel well and outer wheel well with body roll from going over speed bumps, cornering, etc. The sway bar cleared that up, but I don't have any extra room. My car sits pretty low though.

In a 15" rim you pretty much have 275/60/15's or 295/50/15's for tires unless you go to drag radials. The 295/50/15's typically have a 12.2" section width. You won't get those into the stock tubs without also cutting/rolling the quarter lips, or raising the back end of the car pretty high to clear them. Or maybe both.