I just went to the Toyo website and saw that the 275 is 33.2" diameter and the 285 is 33.7". Not too much difference and the 275 as spare would work just fine.I asked Discount to keep the 285 mostly aired down when we tried it, but I didn't visually confirm whether or not they complied. Not sure how much an unloaded tire changes diameter with pressure, I just assumed it might help.