There is a bulletin issued in Feb this year from MBUSA regarding the fuel gauge inaccuracy. My 98 ML gauge behaved badly after the fuel pump and send unit was replaced. It would drop quickly to 3/4 in 10 mils and slowly drop to 1/8 by 170 miles or so. It was off by probably 4 gallons or so. Anyhow, the instrument cluster was replaced but no help, and recently I saw that bulletin from the StarTech website (I paid a one day fee to get in and browse around). I took it back to the dealr and they perform some reprogamming to allow the fuel gauge to read in analogue mode (I think this is what gilly refer to as the actual fuel level inside the tank). This worked for me. I am afraid to post the whole bulletin out because of the legal mumbo jumbo post on MBZ's website. If you would like to read the entire bulletin, let me know.
steve