whoops, sorry, I just saw that you're flying into Miami. Just a suggestion, a sometimes great way to see the country (and I can sense everyone else out there grimmacing :-) would be by bus and train. You'd have to plan out your routes well, but if you did it right, you could get an overnight bus somewhere every other night or so, and you could sleep on the bus. No gas costs, but you lose the ability to stop along the roadside and you run the risk of spending 10 hours next to Marilyn Manson wannabe's.
There's ways to do it if you really wanna do a loop of the whole country, but compromises will have to be made.