Getting 15mpg city in the 4.8i is a pipe dream when that's what I'm getting (barely) in a 3.0si. Well, maybe after a very long break-in as I heard that helps in the fuel consumption department. I'm holding out hope that 15mpg goes up to 18mpg. Maybe with time and mileage you'll get to 15mpg.

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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