First off, you need to look at the section with of the tire and what rim that section width was taken from. Don't assume all 325/50 tires are all the same section width because they aren't.
2nd, depending on the size of the tire and where the tire sits in the tub (read that as the cars ride height) the room you have to work with may change
3rd, If you plan on doing anything other than straight line with the car you may want a little more than a 1/2". Your suspension type and stiffness will play some roll in this as well.
I can tell you what I run and it pushes the mimits to the max. They rub at times both on the inside and outside as the tires are perfectly centered in the wheel wells
28x12.50x15 on an 11.25" rim.
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