Does anyone have slugggish acceleration on their 2010? does anyone have a gas tank that still has 2.5 gals when the fuel gauge show empty?

My acceleration is adequate, but I have not been real impressed with my fuel mileage. The best I have done to date is 400 miles on a tank. I have also noticed when I fill it up the appears to be 2.5 gallons still in the tank, when it telling me the tank is almost empty. I have had my 2010 prius for about 2&1/2 months now. My wife and I like the car and we do get better milage than in our previous car
(2004 Cavalier). If you get any answers about the gas gage accuracy please let me know. I may be getting better mileage than I realize.