I've had mine for less than a year and about 14k miles.

Best tank = 26.59 mpg (mostly highway, some city)
Worst tank = 16.15 mpg (pretty much all city)
Lifetime avg = 22.02 mpg

I keep a spreadsheet and record all my fill-ups. I have yet to have a tank that is all highway, most of my trips have been in the 300-400 range, my best tank range was 580 miles.

I haven't driven a 50i. The torgue in the 35d is nice.

For comparison, in my 06 X5 3.0i, my best tank was 20.8 mpg and the worst was 12.12 mpg with an average of 17.85 mpg over 65k miles with a lot of highway driving.

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16 X5 50i


prior:
12 X5 35d
06 X5 3.0
06 Z4 3.0