I am running 18s and 20s on my monte. 245 40 18 on an 8"wheel, and 285 30 20 on a 10" wheel. I have custom offset rear wheels, and a pretty tight rear suspension. On my car, there is absolutely no more room. and the tire sidewall on the rear is a little short for most people. I would recomend staying around a 9-9.5" wide rear wheel, and a step down on the width for the tire as well.