I think the stock size is actually 225/55R16, but when you use a larger tire like a 245mm width it makes it that much worse because of the stock wheel offset, The 245's on the back of mine are probably about 1" wider overall than the 225's i have on the front. *FiveOhpatrol is correct here:* Also in '91 when they switched to pony wheels, they changed the fenders for more clearance (lip was moved up and fender lip widened i believe, but i may be wrong) and the K-member is actually 1" wider (on each side?) to allow for more clearance and i've heard that it's also a lighter K-member. Not much you can do to solve your problem, if you changed to the stock size the problem should be less severe, but i've heard of the pony's basically rubbing on all pre '91 cars (Ford is cheap so they had to change stuff for a good reason). You should be fine though as long as you don't find the need to take a 90* corner at 60 mph.
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'84 Mustang 5.0 LX
My car