Hi there and new to this forum. Just picked up a 2010 cts premium with 18x8.5 fronts and 18x9 rears I believe. Want a spare and not the inflator in the trunk. Need to remove the tub I read to get a full size spare to fit in the trunk? Question is i want to buy a used cts rim and put a used tire on it. Question is do I buy a 18x9 and will it work on the front as well or buy a 8.5. Just want to make sure I buy the right wheel that will work on all four corners. Thanks.