Diffrent tire sizes?

[Moezer]

Hey guys, I know that running different tire sizes is not good for the x5, but for some dumb reason I thought that my spare on the x5 was the same as the rest of my tires that are 255/55/18 the spare is 235/65/17

I put the spare against the x5 to size it with the 18s and the spare is a bit bigger then the 18s that are on the x5, how bad is that to run on the x5? I have had the spare on for the past 2 to 3 weeks almost, would any thing have gotten damaged in that time?

If it is really bad would it be ok lets say if I get another flat to run the spare just to get me home or to a tire shop? normally I can plug it my self unless the tire blows up

Thanks