Rootbeer's answer that a 275/60/17 & 275/60/20 would be the same overall size is wrong. A 275/60/17 would have 693 revs per mile while a 275/60/20 would be 630 revs per mile. The difference would be big enough to require the computer to be reflashed. If you have a tuner you could change your self.
You find a 20" tire that would be the same size. A 275/45/20 would have 697 revs per mile - close enough.