Widest wheel and tire combo on 15x8 Ralley

It's been a while since I had a Duster but assuming your specs are "normal" whatever that is, about 4.25-4.50 for backspace. You should be able to fit a 275x60-15.

If someone jumps in that knows more than me - which is pretty much everybody - follow their advice instead. I would recommend mocking up what you buy and measure your fit before mounting your tires. There is a fair bit of slop possible back there. If you mount the tires the rims are not returnable.