I have 275/50/15 on my 90 mustang with a 5" back spacing,the problem I am having is that the tire is hitting the lip on the rear quarter panel when I hit any bumps or dips can I just change the back spacing to 4"(there is room in the back side of the tire to get it closer to the frame.

 

That is a lot of tire for a Fox Body.

Take a look under "Fat meats" of Choosing a new tire thread. Don't go changing your wheels before you confirm the room to do so.

Jazzer

 

well found out it was not 5" but 4.5 back spacing,so I went with a 5.5 back spacing and fits pretty good, now I'm looking for the fronts Would like to go 4" wide x15 but need to know whats the best tire size.