The tank is shaped odd. The top of the tank is big, wide. About halfway to the bottom of the tank, it's shape changes to a tall, narrow shape. This accounts for why the sensor reads full most of the time. The tank is not square, so at "half", it may only have a gallon left, but from half to full, it may be another 5 gallons. My car is like this. Run 300 miles from "F" to "1/2", and 50 miles from "1/2" to dead.