2004 cadillac sedan deville gas gauge problem

You can select several displays by pushing the button on the gauge. One tells you the number of gallons that remain in your tank, another tells you the range (how many miles you can drive on this tank at the rate you are using the gas). Maybe others but these are the two most important. Your car has a tank of approximately 22 gallons when full and will typically get about 15-18 Miles per gallon for everyday town driving. If you are doing more highway or interstate driving you will get closer to 20 MPG. Hope this helps you.