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Most FWD cars have smaller tires and work fine even though their front end is like our rear end. The whole point of a spare is to just get you home. For a more extreme example, some people weld their rear diffs to have an "always locked" rear end. When making turns the diff doesn't explode, it just chirps the tires as one travels farther than the other so having a smaller tire will essentially do the same thing, since one is travelling a shorter distance than the other tire. Not ideal, but not worth dropping $200+ for a 5th tire and then trying to stuff an over-sized spare in the back.
