Tough to say, I can't tell what engine you have but my 4.4 averages about 16.5-17.5 in town as long as I behave and about 21 mpg freeway. If your rolling a 4.4 as well I Would think about the pre cat O2 sensors if they have 80k+ on them.

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2013 X5 xDrive3.5i Premium
2002 530I Sport Package
2001 X5 4.4i
2001 M3