Defective sending unit in the tank. When the sender "disconnects", it indicates (--) on the display. Grounding indicates E and a 120 ohm resistor placed in the connector will indicate 16 or F. The sender used on the digital dash cars are rated at 120 ohms full, zero ohms empty, while the replacement analog sender available is 90 ohms full.
So, if you replaced your sender with the currently available aftermarket sender, a full tank would read around 14 gallons. But empty would be accurate.
Replace the sender, that's the culprit. I have repaired and upgraded hundreds of digital dashes for the Regals, and have yet to find a problem with the fuel display circuit.