I'm not an expert but I don't believe the gauge itself should be grounded. I believe you have a switched wire coming in to the gauge then a wire from the other terminal on the gauge to the sending unit & the sending unit is grounded to the tank by the mounting screws. Are you sure you have matching sending unit & gauge? If you have a 30 ohm sending unit and a 90 ohm gauge you'll never get a correct reading.

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