After reading this thread I found myself with 47 miles to empty while passing a gas station which offers a cash discount, so I grabbed a $20 bill and put in 5.5 gallons. When I drove out of the station the truck said 100 miles to empty, so it gave me about 10 miles for each gallon.

 

Not sure if this relates here but my friend used to work in the tech department at chevy. He told me on the computer would display you have less gas than in the tank. He said this is to scare the driver to go to the gas station before running the pump dry and burning it out. The pumps are cooled and lubed by the gas. Running one gallon is going to start hurting the pump. Kind of sounds like the same thing.