This is a common problem in the 00-05 caddy's. The floater gauge in the tank is bad. A somewhat involved repair, as the tank would need removal to replace the floater gauge. Many owners rely on the "GAS CONSUMPTION READOUT ON THE DASH". Reset it at each fill up. Once the 16 gallons have been used, fill it again and reset the tabulation. Run out of gas once or twice and you'll learn how to use the computer readouts like a pro!