Sounds like the fuel sending unit is junk. Mine on my 00GT does the same thing. I go by my trip odo. I know once I hit 400mi. it's time to fill up. Pretty sure these cars only have a ~17gal. tank. I've never filled mine from totally empty before though.
What type of driving do you do? Mostly city? Combo of hwy and city, or mostly highway? That and your driving style will impact fuel mileage, among other things.
Tires properly inflated? How many miles on the plugs/wires? Clean the MAF and replace the air filter lately?
