The fuel gauge is horribly inaccurate.
I wish the Agility had a trip odometer, they make good fuel gauges once you know how far you can go on a tank.
When mine hits the red zone, I typically fill it up and it holds less than a gallon. If I just ignore it and go another 40k or so, it will hold about a gallon.
Most I've ever put in was 1.1 gallons, and the needle was buried way down at the bottom of the gauge.