Since the question of fuel economy still comes up, I figure this may be useful for anyone that currently has a X1 w/ the larger N55 engine (xDrive35i, turbo I6) or is considering it versus the smaller N20 engine (xDrive28i, turbo 4). Granted, this is only my experience but it may serve as a frame of reference to others.
The short of it all is that, last year, I started tracking the price of gas every time I filled up. Why? Primarily because I grew curious how wildly the price would swing. I've always noticed some fairly large fluctuations and was curious how it trended over the course of they year. I kept doing this in 2014.
However, I took delivery of a new X1 35i "M-Sport" (oh, how I hate that marketing) shortly after the new year.....ours, not the Chinese.
I was curious how closely the OBC's readout for mpg tracked to manually calculating it so I started tracking this as well.
I average 250-300 miles a week and commute everyday except when I am on business travel. My 46-ish mile (round trip) commute is a mix of city and highway driving. Weekends is mostly city-ish driving while I run errands. Generally, I don't drive like an @$$ but will get on the throttle once the oil temp is warmed up. Until then, I keep it under 3k RPM most of the time.
During nicer weather (summer through fall), I tend to average in the 23mpg range. It would be a little higher (24-26mpg) but, as expected, it drops on the weekends while running errands (city-ish driving and stop/go traffic).
I've also noticed that fuel efficiency dips a bit after the vehicle sits for a bit while I am away on travel. This caused some of the wild variation (drops) during break-in (re: dips from January thru spring/early summer) so the gradual increase in fuel efficiency is a little broken up. We also had a long cold winter (takes longer for engine/trans to warm up) and had a real cold spell in early/mid Nov.
Something else I noticed is that the OBC readout for mpg is typically within a couple of tenths of a mpg when compared to hand calculations.....nothing that wouldn't be within the realm of the margin for error (OBC rounding and amount of fuel burned vs added to the tank).
Overall, I am happy w/ what I get fuel efficienty-wise. It is, after all, a 300lb-ft/300hp, 2-ton beast.