FYI- I read somewhere that the tank level indicator needs to move by 20% for the computer to consider tank level as legitimate. If you drive and drive and the tank level never goes down ( because you keep topping off ) the CPU considers the tank level indicator to be "stuck" and flags a trouble code, maybe a check engine light too, but I forget the specifics... other than the tank level must move in a certain amount of time/miles.

You CAN get really long tanks ( 600mi + ) by over-filling your tank.

The 15 gallon tank is "nominal" tank size.
There is about 2.5 gallons of air space ( or more ) in that tank.
I have been able to shove 17.5 gallons in there once or twice by parking on a hill with the fill tube on the highest side. That will get you a 600+ mile tank for sure!
-John