Strangely enough I tested mine yesterday:
Same 10 mile journey, A roads, B roads, steep country lanes etc, 1st run drove like a tw*t: 2nd run very conservative, followed the displays recommended gear selections, kept the rpm's as low as possible. (there was a reasonable amount of weight in the boot for both runs)
1st run 43.8mpg - 36.7mph average
2nd run 49.5mpg - 35.5mph average
So (correct me if im wrong) based on a 60ltr tank, it eqautes to roughly a 68.4 miles per tank extra driving as per the efficient dynamics gear selections and being as conservative with the power as possible.
The car has only done 3k miles so far. I dont think my 06 320d E90 was much more efficient, I still seem to be getting the same overall miles per tank, say 550ish?
Hope this helps