2 Gallons In The Fuel Line?

I do a lot of Highway driving, with the cruise control for extended periods of time. When the fuel gauge reaches about 1/5 from empty a warning light comes on and the "distance to empty" display flashes (this commonly happens when it reads around 40 miles to empty). After a while of driving, the "distance to empty" number eventually goes blank. I began to add the flashing number to my trip mileage, and stop for fuel when I reach that mile. (Example: Trip meter is at 360, distance to empty flashes "40", so I make a mental note that when my trip meter reaches 400 I need gas.) Each time I fill up I have to stop at around 18-18.5 gallons. My car has a 20-gallon tank. In theory, it would be safe to assume I could easily get another 40-50 miles of highway driving out of it.