The best figure I have ever reached with an E320CDI was 5.0 l/100km, about 56.5 m/imperial gallon. That was very smooth driving on a highway only, at about 60 mph.
A 220 diesel would certainly do better (was it an auto box car, mainly a difference at the airport driving), even if it was older than the E320CDI. For very smooth driving, the figures probably are pretty accurate, not just an optimistic figure from the instrument cluster computer (on my W221 the IC is even a bit pessimistic but surprisingly accurate while I've seen some gasser Toyotas being way too optimistic).