I don't even remember the last time I had to change a tire. I owned some cars for years without ever having to get flats fixed, or the tires changed. Unless you drive in very remote areas... and even then with an auto club membership and whatever stuff in a can comes with the Elio, you should be able to make it to the next tire place.
Using a front tire in the rear might void your warranty should anything happen (if it is even possible), is probably a terrible idea for handling reasons anyway and your fuel economy will suffer in a big way if the spare is carried on the outside.
I have done motorcycle trips of thousands of miles on partly very bad roads (without spares of course) and never felt the need to carry replacement tires. Just make sure that you leave home with good tires, as flats seem more likely when tires reach the end of their lifespan.
