Greg,
I did a lot of dealer swaps when I worked at dealerships years ago and then as an 'independent' to pick up extra cash. I learned the hard way not to trust fuel guages. I once took a new car to NYC from NJ, and stopped along the way to pick up the lot kid, who dropped an identical car at another dealership. The fuel guage in the car I was driving read half full. I happened to ask the kid why he put so much gas in this one, and he said he didn't put any gas in at all, because it was half full when it came off the truck. Just as he said this, the engine died and I coasted to a halt on the side of the East River Drive in Manhattan....not a fun place to run out of gas!
As far as driving for fuel economy is concerned, I've always been easy on takeoffs and braking, and get close to or above the EPA rating on our cars. I'll just cough up more money at the pump, like everyone else.
Harold