If you're getting 17mpg in an X6M constantly, you should stop lending your X6M to your 125 year old grandmother!
My X6M was recently getting 99.99 MPG and has since gone down to 37.1 MPG. This is how I know my MPG meter needs to go to MPG meter heaven because it's broken.
Driving on backroads I was able to get a personal best (or worst) of about 9.1MPG - haven't got it into the 8's. This was driving in M mode and manual shifting, driving pretty hard. Usually I'm around 10-12mph before my MPG meter went sideways and decided I'm driving an i8 instead of an X6M...
The only time I suspect you can do consistent 16mpg or higher is if you leave the car in D mode (it upshifts through the gears so you're in 6th gear at 40mph) and have the Power (maximum horsepower) setting disabled.
In other words, not fun at all!
You can replace "X6M" with "X5M" in this post and all still applies.