You might what to take some readings on your fuel sender as a first step. With a full tank it should read 90 ohms, half tank is 45, 0 is empty. See what kind of results you get. Taking a guess right now it sounds like the calibration resistor on the back of the meter but without some measurements, I can't be sure. The resistor is a faily easy fix if that's it. http://www.chevelles.com/techref/ftecref18.html
If it was me, I'd take the dash pad off. If this is a sweep dash (non gage dash) the fuel meter is fairly easy to get at. The sender wire is the tan wire. Need to unplug the connector from the meter to read the sender.
don't take the dash pad off just unplug the tank unit wire at the rear bumper turn on the key gauge should go past full if does bad tank unit or tank unit ground. p.s. make shure you don't unplug license light as it is in the same area
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