1997 Buick Century Gas/mileage problems

I have a 1997 Buick Century (3.1) engine and the fuel tank capacity is rated at 16.9/17 gals, but I'm finding that it may not be the case or is it? I filled up the car the other day when it was at or below the red hash mark on the fuel gauge after driving and feeling the engine sputter. I subsequently fueled up and it only took 14.001 gals to fill. What gives? Also, the advertised mileage based on the books and specs. says City: 20/mpg, hiway: 29. I'm no where near that. I've had the head gasket replaced, tuneup with double platinum plugs, new wires, all fluids changed, fuel system cleaned and new fuel filter. Also, new gas cap for the car. The new EPA standards advised that the mileage estimates have been revised downwards, to 17mpg & 25mpg respectively but I'm still not near the 17mpg. Could you tell me what to do to increase the mileage I get from a tank?