If you unplug the sender, the gauge should read well past full. If you ground the brown wire from the sender, then the gauge should read empty. If that is the case then the problem is in the sender unit, sender grounding or the wire from the back to the front. The sender actuates the dash gauge by "grounding" it in effect, so if the sender is not grounded completely then the gauge can end up anywhere.
An easy location to do your gauge tests is at the fuse panel as the wire from the sender plugs into the panel.
Jim
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