well, you will hear many stories on this, but i will see if i can help you out some. if you are going to run anything other than a Nitto, 275's will be about the max on your 8.5 wide stock rims, but if you do run Nitto, you will be able to fit a 295 on the stock rim...Nitto runs small, a lot smaller than most tires...i have seen 275's on 9" rims that swell more than my Nitto 295's on my 9" rims...
A 255/45-18 will fit and be .7" wider and only be .3" shorter than stock. Pretty much all the other sizes will be much taller or shorter(more than .5") and probably require at least a 9" wide wheel.