the all controversial MPG arguement

This is a question that has been going through my head since I started my build, Ive always envisioned taking my car on long trips across the country if I ever get it finished. Hence the extra money on the tremec 5 speed and the fuel injection, mileage and reliability. But I have no idea what i can expect for mileage out of a 500 horse 408, only a 16 gallon tank.. not a huge deal in Cali where theirs a fuel station every 20 miles but in the mid west you can go 300 miles before you see something especially if you need 91. Im hoping for 15-16 at 80mph that's the interstate limits up here but i really dont have a clue...