Largest rear tire size

I did my 5 lug swap with stock fox length axles with 17x9 FR500's from Late Model Restoration. They have 24mm offset 5.85 backspace. I run a 275/40/17on the rear and 245/40/17 on the front. I had to flip the quad shocks to fit them but no other mods. It has about a 1/4 inch from the tail pipes. Only thing I would do different is going to 255/40/17 in the front.

There are a lot of variables involved, and even if you know backspacing, tire size, etc, it still may or may not work for you. Production tolerances vary and ACTUAL tire width varies, to it's easy for one car to have 1/2" clearance and another car to rub, even though on paper they have similar setups.

Bottom line is, if you want to run a 275 or wider tire, you have to accept the possibility that you may or MAY NOT be able to run quad shocks. There are LOTS of guys who manage to run a 17" 275 and keep their quads, but there are also lots of guys who couldn't. My car for example, there was no way in hell I was going to get a quad to work with my 275s.

Something else to consider, is the less backspacing you have, the more quad shock clearance you gain, BUT you also push the tire closer to the fender and then you may have to roll the lip to avoid slicing your tire. This magnifies on how much your car is lowered.

Also:

http://www.stangnet.com/mustang-forums/716968-wheel-backspacing-offset.html

Here are some pic's.

I have drop springs on my car, and I am running 17x9 fronts & 17x10 rear 10th Anniversary wheels on my 91 GT. Tire sizes are 245/45 front, and 275/40 rears, but I removed my quads.