1999 Buick Regal Dashboard mode "Distance to Empty&quo

Electrical problem
1999 Buick Regal 6 cyl Front Wheel Drive Automatic

The Distance to empty readout doesn't give a proper readout. I filled the car up at a little over 1/2 full on the fuel gauge and it took 8 gallons. The DTE readout said 125 Miles. It used to read when full around 320 miles. Do you have any idea what's wrong? Note: This readout does not have a reset function.

Thanks Very much on any ideas or thoughts.
George G