First is to add a ground wire from sender mounting screw to frame clean spot. You can just run temporary test wires all the way from sender flange screw to battery ground post. Run it along outside truck if you want. Then same for gauge body. 80% of issues with gas gauge problems is ground.
Do the testing as described on internet for testing sender/gauge.
Gauges can be tweaked both electrically and mechanically (needle) if that's all that is wrong.
"Always read wrong" ??? sender/gauge old or new? Never inspected? Never removed by you? Sure not wrong aftermarket parts/wired wrong by previous owner?
Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn
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