Sorry, don't have the N52 in an E70, but do have it in an E61 wagon equipped wit it which is probably not all that dissimilar in weight. The N52 does not get great mileage. We get 17-18mpg combined and 20-22mpg highway. Those figures are calculated, not reported on the screen.
As andrew suggests. Do a calculation on a few tanks. You're probably about 15-15.5mpg actual as I find the screens on BMWs are 1-2mpg optimistic in most cases.
I can see how your N63 gets better mileage andrew, it is working so much less hard to move the vehicle around. The N52 is a pig in these heavy vehicles, my E61 takes A LOT of pedal to get it motivated to move. All our other Bimmers feel like rocket ships comparatively; e53 4.8is, e70 35d, and f15 35d. My N62 in the e53 gets almost the same mileage as the E61 wagon.
