Depends on the width of the tire. The sidewall height is expressed as a percentage of the tire width. I brobably wouldn't go any lower than 40 on a 245 or a 255. The shorter you go, the larger gap it creates between the fender lip and the tire. Also, the shorter the sidewall, the less the tire can "give" over rough roads, and you are also more volnerable to bending a rim on a pothole. And last but not least, a shorter sidewall on the rear is crap for any form straight-line traction.
Thanks. I didn't want for the rear, I wanted a set for the front. Since(when) I get new wheels I want to get bigger wheels out back, I don't want the tire height to compensate for the inch that is bigger in the back. I like the inclined look.
What size tires are you wanting to put on the back? If you go with 315/35 on the back and 255/40's on the front the front tires will be shorter than the rears. That's what I'm puttin on mine