On Saturday, I fueled up for the 4th time in my Prius. After the nozzle clicked off, I put the cap on and drove away. I noticed that the gauge only registered 9 pips, or 1 pip still unlit. Thinking I obviously didn't fill up, I returned to the gas station and added another .6 gallons of fuel before it wouldn't take any more gas. Again, the fuel gauge showed only 9 pips, leaving one pip unlit. What is the problem? P.S. I filled up after my range registered 0 miles left, filled up with 7.2 gallons the first time and added the .6 the second time.