Fuel guage question

[LM44S]

After replacing the fuel gauge in my 38 pickup it registered full when it only had a couple of gallons in it. New tank and sending unit. Understand that low voltage could be the problem. Checked it out and have 6.3 volts to the gauge going in and 3.8 coming out. Does that indicate a faulty gauge or do I need to add a ground wire somewhere? Thank you for any advice.