I have recorded all the mileage for all of my BMW's.
For my 2009 135i (N54) I averaged 11.25 L/100 km upto when I traded it in at 48,490 km. The engine was plagued with injector and carbon buildup issues so I don't know if this is representative. The travel was a mix of highway and city with a bimodal distribution ranging between mid 12 L/100km when dominated by city driving and mid 8's when on the highway.
For the X5, the data is not as clear cut - the tank is much bigger so I never record a city or highway dominated fill up and thus the range is much tighter but I can get a very consistent mid 8's on the highway and a good mid 10 in the city with an overall average of 9.9 L/100km (8450 km). This is recording the amount of gas and distance traveled - not using the numbers from the on board computer which over estimates the average fuel mileage. One added benefit is that diesel prices FALL
in the summer when premium gas prices RISE dramatically.