I have replaced the entire fuel system on my '63 Falcon. I put a new 16gal fuel tank into the car, and the matching sender. My fuel gauge works, and passes the factory manual tests of grounding the sender wire/etc. The coolant temperature gauge also works (and seems to be reasonably accurate). So, here's my problem:

My fuel gauge shows full when the tank is full, and slowly moves to empty once I've use a whopping 5 or 6 gallons of fuel.

This past week I drained the tank, pulled the sensor, and confirmed that the Ohm readings across the entire range match the specs. The float is free to move around correctly. Hooked the sensor up to the car, and with my girlfriend watching the gauge we worked out the gauge does not begin to move until the float is roughly 2/3rd "up", and that's when we see just below empty. And at then the gauge sweeps to full as the sender does too.

So...what now? Replace the actual fuel gauge, vibrator, or both?