MotionX GPS Drive needs a cellular Internet connection to see the underlying maps. It will still track via assisted satellite GPS if you happen to drive outside a cellular area but from Miami to Orlando I would suspect it would not be for any amount of significant time if at all. In Miami and Orlando itself you'll always have a cellular connection.
There are apps by Tom Tom, Garmin, CoPilot, Navigon and Magellan that have their own maps and data stored on your iPhone and don't need a cellular connection at all. However, they have a cost considerably higher than MotionX GPS.
I use the Tom Tom GPS app as I do a lot of driving in an RV outside of cellular areas like national parks and forests and off the beaten path roads.