What is up with the Prius' gas gauge?

I fill up when the last bar starts blinking. usually less than 10 gallons. Filling up with two bars is being a little too cautious.

As an aside, It seems the MPG shown on the dashboard display is somewhat misleading as it shows more MPG than I am actually getting (by measuring the amount of gas it takes to fill up and dividing the gallons by the miles I drove on that tank)

 

My car has a similar problem. I usually fill my gas up around 80 miles/ 2 bars. My hybrid battery ended up dying and it was because i was running out of gas but my gas gauge was messed up. I don’t have as much gas as my car is telling me. Is there any way to reset the gas gauge or to fix it?