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don't know the exact measurement, but I know how to find it.
Take your wheel off, and take a straight edge of metal or wood to go in it's place. Then measure from the backside of your straightedge to the inside of the wheel well and find the first thing that it hits. maybe 1/2'' to 3/4'' spacing from there is the most you can get. Then measure from the back of your straightedge to the outside of the wheelwell. Add these two and that's the widest you can go. Oh, and for the front wheels, you might have to get a bit creative with your straightedge. Make sure to go lock to lock on the steering and measure what it's gonna hit first in either case.
I've got 255-70-15 on my '87 F-150 (see my gallery) and they work with no problem. In fact, I had even wider ones on the back, but I didn't like the way they looked - they were more squat than I liked. Still, no problems.