OP, 13mpg sounds about right for our CA winter blend and mostly city driving in SF. I have 147k on the clock on our '04 and it's averaging a bit under 16mpg with 60% Highway. I even rebuilt the vanos seals at ~90k and just renewed my cooling system (T-Stat, waterpump, expansion tank, hoses) 2k miles ago.

Oh...I also just passed SMOG without issues

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2004 X5 3.0i
Born 02/04
Delivered 03/04