I might be wrong but I think the 2005 X5 3.0 is 16 mpg (city) and 20 mpg (hwy)
I would assume it would be the same if not worse for older model (i.e. 2002)
This is the MPG you should get when the car is brand new with fresh plugs/oil/coils/filter etc...
If you're driving in the city and getting one less MPG then that is acceptable. Also check tire air pressure, might give you 0.5 mpg increase or maybe more lol

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2005 BMW X5 3.0i
2013 BMW M3 Competition