Rear tires are pretty easy. You're really only looking at vertical travel, a little lateral movement with the springs, body roll, and sidewall flex. 1/2" is usually a good guideline, you can get away with less to the springs, but not usually to the quarter.
The front is a lot more complicated. The tire to frame clearance is easy, as long as the tire isn't hitting the frame at full lock you're good to go. Unless maybe you're building a drift car. Otherwise, the only time you'll be at full lock is at parking lot speeds, and even if you do get a rub on the frame when you hit a pothole at full lock you're not likely to hurt anything.
The fender is a whole different ballgame. I'm not sure I even want to give a recommended distance. It depends on body roll, ride height, your car's alignment, your cars amount of suspension travel (which depends on ride height and torsion bar size), how much travel you need with the wheels turned, etc.
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very rough measurement is usually about 3/4" from the tire to the inside lip of the fender. But, like I said, your experience may vary.
Also, it depends on tire width AND height. 255/60/15's are 27" tall. That's just silly on the front, you'll have nothing but problems and there's no reason or advantages to do it. On the other hand, I run 275/40/17's on my Challenger without issue. They're 25.7" tall, and my car sits pretty low. If you stick to tires that are ~26" tall, you can go with a 275mm wide front tire with the correct backspacing. But you'll probably need to run 17" or larger rims to make that happen.
My Challenger has 275/40/17's all the way around on 17x9" rims with 5" of backspace. They barely clear the fender lips, but have a little room to the frame. But I also run -.9* of camber and +5* caster, and my car sits fairly low. It also depends on what you're willing to do. I removed the inner fender braces on my car because of the ride height, I wasn't clearing them at full travel. But I haven't really done any work on the fender lips, the driver's side is stock and never rubbed, the passenger fender is partially rolled because of some occasional rubs when entering driveways (ie, wheels turned + suspension travel). If you weren't opposed to rolling the fender lips and got your backspacing perfect, 285's might be possible without cutting/flaring. Without any mods I would say 275's are the biggest you can safely go, and even that will depend on ride height and suspension choices. I run 1.12" torsion bars and am lowered up front, so I have less suspension travel than a stock car.
