i have 2 17x9 cobra r's. they have 255's right now. how wide of tire can i go on the 17x9"s? and I was thinking about getting 17x10-10.5's in the back, how wide can i go without rubbing?
Many people run a 315/35/17 on the rear with a 17x10.5 wheel.
You'll probably need to flip your quad shocks (very easy to do). Some have no rubbing at all. Some use a 1/4" spacer and have no rubbing anytime.
I'm going this same route eventually, but I've got concerns with my 3" catback pipes. I'll have no quads by then, so that's gone. Using a small spacer is the route I'll go as well to get the spacing just right. If you're lowered agressively, there might be a rubbing issue there too, but anywhere from stock to about 1.2 inches seems to be rub free territory.
275/40 is perfect on a 9" rim. You can get a 17x10.5" for the rear in the Cobra R with 315/35's that should fit no problem, lots and lots of guys on here including me run these sizes. Whether or not they rub the quad shocks is hit or miss, they did rub on mine and I had to remove the quads. (made next to no difference BTW)