possible fuel gauge malfunction but its a chevy.
over the past year chevrolet fuel pump failure has increased by over 900% the fuel level sending unit is incorperated in the fuel pump module,
these fuel pumps do not like fuel additives nor do they tolerate E85 fuels.
you will need to replace your fuel pump assembly, try to locate a carter brand pump they are very reliable.
as far as replacement.
just open the fuel filler door remove the two torx screws and one push pin from the fuel filler neck, go under the truck at the back of the bed on the frame cross bar and unplug the bed plug, remove the 8 bolts that attatche the bed to the frame and the one ground wire under the bed by the fuel filler neck. get a budy and lift the bed off of the frame, now you can access the top of the fuel tank for the pump replacement. dont get discouraged it is a easy fix, i can do these in under an hour.
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