Usually the rule is 2" wider than the rim width.
So 255 is the max recommended. In fact, Ford put 255/45/17's on the 94-95 Cobras with 17x8's, but then stepped down to 245/45/17 for every other 17x8 setup.
But guys have stretched wider rubber on there. I've seen 315's on a bullitt wheel. But with the bulge you start getting sidewall flex and the car will actually rock on the sidewalls if you push it from the side.