We have the same truck; mine has the 19.5 gallon tank (source: owner's manual). my gauge is also 'wrong' and has a few gallons even when on E. I don't think the gauge is broken, but rather it is designed to read less so people will see the E and get gas BEFORE they actually run out and get stranded lol. Like a scare tactic.
Like yours I will be on the E and fill up about 14-15 gallons. Even when the little light comes on and it beeps I still have 3 gallons or so.
My needle also moves both directions .. If I drive fast enough on the highway the needle will actually go UP lol
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Dennis Brady
Black 2001 4x2 XLT Supercab 3.0 "Shadow" - WIA
Black 2001 4x4 EDGE Supercab 4.0 ('Body donor' for Shadow lol)