W220 has fuel tank (A2204707601) with 88 liter (23 galon) volume, unless vehicle has option code 282 (SKI-BAG) or 308 (REFRIGERATOR IN REAR) in which case the fuel tank (A2204707701) has value of 78 liters (20 galons).
Fuel tank (88l)
Fuel tank (78l)
The 88 liter tank uses 11 liters as a reserve. The 78 liter tank uses 10 liters as a reserve.
The reserve is what is left when the needle is pointed all the way down to the bottom of the yellow area and the fuel warning light comes on. Hence, the fuel gauge in the instrument cluster shows the fuel level minus the reserve volume.