a 295/55r20 is not 12.5 wide, its actually 11.6. and a 285 is 11.2. that's based on converting numbers
but ya I think its just the added height that makes the difference. Its not that much and I really have to lock it when in reverse to make it rub. I would have thought the 285 would clear no problems based on people running wider tires and no rub, but I guess not. I think the .25 spacer should take care of it though.