You need nearly zero resistance between the body of the sender and engine block and neg' battery post. Too much teflon tape on the senders threads, etc,, can increase resistance.
A better guess... the gauge voltage limiter is the problem, your fuel gauge reads low also although you dont know it. In most early panels the limiter is inside the fuel gauge or temp gauge. that gauge has a slither of metal on the back of it that provides a ground for the limiter to the panel. Panel mounting screws link the ground path to dash frame. So its a daisy chain of metal to metal contacts from one end to the other.
Other members have pulled the fuel gauge to clean the contact or simply snugged up all the gauge mounting nuts that have loosened over time. Limiter and gauges work right again.
Adding a dedicated ground wire to the instrument panel insures a good ground there and can even make dash lighting bright again. Hope this helps
