65 Fuel Gauge Question - Stumped

I recently finished restoring a December build 65 911 only to have it run of of fuel with half-tank showing on the gauge. I replaced the sender with a known good one only to show the same faulty reading (full on the gauge with the tank only half full). I plugged in another gauge only to get the same faulty reading. What am I missing? The gauge will drop from full to half when in fact the tank starts out only half full. I do notice that with other cars the fuel level needle fluctuates on hills and through corners, whereas this needle is steady going downward as fuel is consumed -- regardless of the sender used. Thanks in advance for any help.

John, when I am testing that system I take the sender out of tank, plug on the harness, then turn the sender upside down and check to see what the gauge is showing, then back to right side up. See if gauge reads both extremes. Make sure you've got the wires plugged on the gauge correctly.

Thanks Ed -- good tip regarding removing the sender and testing extremes. I'll recheck the gauge wiring - green to G, Blu/yellow to + on the gauge, Black to low-level light. I'll also check brown ground wire that comes directly from sender -- I assume it goes to gauge body, or does it go to low-level light?

Both the black and brown ground wire go to the insulated bulb holder of the fuel warning light. Green goes to 'G' as you said. In addition there will be another brown ground wire that goes to the gauge body.