I talked with the tirerack a couple days ago about this. The 19 option is 255/60 19. Right now there is only 1 tire choice...the RS/A's. Tirerack told me this will be changing in the next 6 months. Other manufacturers will be coming out with options for us in this tire size. Right now Goodyear has the lock on the market...I believe not only on the 19s but the 18s as well. So if a current owner has to replace a tire...it's either the RS/A's (19) or the Forteras (18).
I had the same situation with my 2003 G35. When it came out I only had one tire option it the size the car came in...after 60,000 miles when I went to replace them...I had about 9 choices.
Did everyone read in one of the big magazine articles that the Acadia on the RS/A's stopped shorter than the Porsche Cayenne (!!!!!) and BWM X5!!! The RS/A's...while not great tires in any one area...are average in every area. The Cayenne/X5's were supposed to be better and sport oriented driving and the Acadia...with it's huge disk brakes and awesome software stopped shorter! Huge cudo's to GM for designing such incredible brake and tire combo's for this ride.