So before we even picked up the car we swapped our 22" platinum wheels for 20" Fuel Mavericks with 275/60R20 (33.03") Cooper AT3s in SL. We have no rubbing issues at all. LGE-CTS went to 35s with a lift and different bumper and some trimming, but I don't see why 275/70/18s (33.23") wouldn't be a problem. We get about 20mpg on the highway with ours but it's relatively hilly PNW and driving 65-80mph. The 10 ply tires will kill that MPG though coming from a 4-ply with all of that extra rotational weight.
View attachment 24817