I have 15x7" cop rims on the back of my '74 Duster, they have a 4.25" backspace. Not a whole lot of room to the spring, I'd say no more than a 1/2", and I only run 225/60/15's. I would say that 4.5" of backspace is as much as you can comfortably run without a spring offset or spacers. Some cars might be able to get away with more, but it won't be much more.