18x9.5 +12

In the back it's because you've got a Dart. The Dusters/Demons/Dart Sports have a full 1" extra space going to the quarters. Even with the 1/2" spring offsets 275's can be really tight on the Darts.

In the front they should fit fine, I had very little issue with the 275/35/18's beyond the front lower corner of the fender. The fender roll was only because of how much I've lowered my car, and like Craig has shown the only rubbing I really ever got was going in and out of parking lots- bascially the combination of having the wheels turned and suspension compression at the same time. Since I've rolled and pushed the fenders they're well clear. I've considered going to 285/35/18's, but I'd need new rims up front since the 18x9's are too narrow for that.

It's pretty obvious that in the rear I wouldn't be able to fit more than a 275. The front on a +12 wheel surprised me. I didn't figure that a fender roller would stretch the fender out that much. Although, my car is pretty low as is.

I'm thinking about upgrading at least tires in the near future, if not wheels too, but to play it safe I may end up going with a 265/35/18 as I want to reduce the likelihood of rubbing altogether.