I log all my mileage on all my cars. I have owned my X a little over 5 years and 22k miles, and I am at 117k total mileage now. My lifetime average since I bought the car is 17.0 mpg. Which is very low. I would not say 17mpg is good for a 3.0 unless you drive all city all the time. Or if you live in California or anywhere else that you have to drive the 0-90-0 dash like an absolute maniac every day just to survive.
The reason my average is this low because:
1. Somewhere around 25% of all the miles I have put on this car is towing a trailer, averaging 12-13mpg.
2. When I am not towing, this is a sparsely-used second vehicle, which I almost exclusively drive on short trips in town. The best I have gotten is 23mpg on the highway, but I rarely can get 20mpg from a full tank.
Here is my mileage and maintenance log. I have found this is a good practice to actually know what your mileage is, how it changes with your driving, and how it changes when things break and/or get fixed. I do not know how accurate the dash is, but they are often significantly off. Fuel-up logs will tell you the real story.
https://www.fuelly.com/car/bmw/x5/2003/bdc/454415
FWIW, I try to minimize my mileage on this car because the mileage is so bad. I prefer not to subsidize the Saudi royal family's next palace.