If it's like a 70 the tan wire going to the sending unit is accessable behind the rear license plate. Pull the connection apart, with ignition on the gauge should go to empty. If not then it's the wiring, maybe grounded somewhere or the gauge. The wire goes up through the trunk then under the carpet and up to the dash.
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Hank
70 Chevelle SS396(454), M20, 3.73
My Chevelle
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