not to be a smartass.........................but............................
the tank's empty when it stops running
honestly, I've never run out of gas but I've driven it with the needle below the line............I think every vehicle's gonna be slightly different..............if your sender arm is bent you could be getting erroneous readings and could run out before it gets to the line..........or the other way, it might not read completely full when it is
the only way to find out is drive it 'till it quits(carry a gas can with you)
Also corroded contacts will give you different readings. The sender send a signal out in oHms to the gauge. If your contacts are heavily corroded the reading will be off.
I believe the tank holds 23 gallons more or less ( cant rember if its 22.5 or 23.5) but somewhere around there. Run you truck to the E mark and fill it up. I did this and only put in 18 gallons. I ran it way below the E and only put in 21. One day I am going to run it out when I have a extra can of fuel just to see how low it will go.