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.