The fuel gauge may read incorrectly. A service bulletin has been released by Toyota regarding this concern. The fuel filler pipe or the instrument cluster may need to be replaced to correct this concern.Google+
Car Problem Reports
Toyota Prius Fuel Guage may not read correctly
Toyota Prius Problem
Average Mileage: 92,955 mi (47,000 mi - 150,000 mi)
Visitor, 2006 Toyota Prius, 135,000 mi
Fuel gauge is stuck at one bar past half. I've filled it to full twice but it doesn't budge. I've driven it 300+ miles, but the gauge doesn't move up or down. I'll not fill it over the weekend to see I it'll go down.
Visitor, 2005 Toyota Prius, 150,000 mi
I put gas in the car. Still shows one bar.
Visitor, 2009 Toyota Prius, 107,000 mi
Not fixed, stuck on one bar, but filled car up and no change...
Visitor, 2009 Toyota Prius, 80,000 mi
ran the battery low , then the gas guage registered 1 blinking light, filled the tank and it registered a half tank ..6 bars and stays that way 100 miles burned.
Visitor, 2007 Toyota Prius, 83,000 mi
Fuel gauge reads about half full when it is filled completely.
Visitor, 2008 Toyota Prius, 47,000 mi
gas tank is full but gas gauge only registers half a tank, recalibrated system and still registers half tank.