Real world mileage for me in a 3.0si is just 15.0mpg according to my trip meter (no manual shift). This is 80% local roads. On these local roads for many weeks, I was about 14.8 then it went up to 15.2 after only a couple short highway runs. Hoping real-world will eventually make it to 16-17, but right now I'm laboring to get there. 2k miles, don't think I'm really in break-in anymore... and not all that heavy a foot either.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr