Stock width Rear end and stock location springs what offset do i need for 295 ti

[MEK-Dangerfield]

My challenger i have stock rear end and tried a 15x8 in the rear with 4.5 BS and it had about 2.25" between the rim and leaf spring so i was going to go 10in wide figured split one inch each way that would equal 5.5 but xv came up with a 5.75 i know im splitting hairs im just trying to get the biggest set up in there

I don't think you have a stock setup if you have that much space between rim and leaf spring after putting a 15x8 rim on with 4.5" backspacing. I had maybe 1/2". 

[cudazappa]

You go too wide and you will rub once you start MOVING the car.

I have test fitted 18x10s with 5" backspace on my old 71 challenger.  I like .5" clearance on either side of the tire.  Those were set up to rub the quarter panel wheel lips.  I might have been able to gain .25" with 5.25" BS.  Not a common size and I'm not about to try with custom rims.

Right now I run a lowered stock suspension layout Challenger.  9.5" wide is as wide as I could possibly go in the rear.  Fit a 285 wide, and that would be tight.  Right now I have 17x9s with 5" BS and 275/40s.  In the front I have 17x9s with 5.0625" BS and 275/40s.  Those are VERRRRRRYYYYY tight.  I'm going to run more negative camber in the front to make more room this year, but I will still rub until I create more backspace (I run bolt on spacers, 1" in rear, 15/16" in front) with a custom 13" rotor kit.  But I can autox without rubbing.  To me that's what's important.  Expect even more rubbing if you have a completely stock suspension.  I have upgraded front sway and t-bars right now.  I'm sure the super stiff springs in the basement and adding a rear sway will also help eliminate the rub I get when I'm at full lock and articulated turning up a hill (from Main St to my parent's access street, and some driveways).

oh yeah, this is a 71 challenger...