Ha I knew it. I was almost sure the old 7 series used to call out this code as well. Yes, such a variation is not much, and I am sure if you get a flat on the spare, you will get a real message on the OBC. Since the X5 is AWD, putting on a different overall diameter spare can affect the entire drive line. I am also sure BMW would not let this be like some old school Lincoln, and built if safeties to prevent damage to the running gear. I am sure if you don't have an option, a smaller wheel will get you home, but I personally would match the overall diameter.