This area has been a bone of contention for some time. Most seem to get somewhere around 25. Some a little higher and some a little lower.
I still would like the defiinative answer why some 240ds seem quicker than others as well.
My red 240d seemed to get about 25 on its long journey home once I remember in the winter. Again accuracy of this was not certain. I also have one that seems better all around but it is not proven either.
Driving habits are going to impact it to some degree. Most important is once you are sure of your speedometer keep carful records over say ten tanks of fuel to quash any filling variables.