I am looking to put a bigger wheel and tire on the back of my 66 Biscayne. I would like to stick with a 15" rim. What is the widest I can go with, without modification? What is the widest tire without modification? What backspacing should I use?
You can not assume that the car you have has not been modified or wrecked in the past that would affect tire size. But statistically over a great number of individual cases an eight inch tire on a seven inch wheel up front, and a ten inch tire on an eight inch wheel in the back are about the biggest you can fit.
I think you should be able to squeeze a 10 inch wheel, backspacing should be 5.5" which will place the wheel in the center. As far as tires 275/60/15 is safe, it will fit like a glove. I don't think you can fit 295 without some rubbing, I could be wrong.