The only thing I've noticed is that my mileage decreases once I hit 70 or more - must have something to do with the drag, but I took a 3k round trip to Cali shortly after I bought my X3 and saw speeds averaging in the 90's and had no problems then nor have I had any since.

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MWBBMW
2006 X3 3.0i Highland Green Metallic, Sand Beige Montana Leather, Steptronic, Premium Pkg, Cold Weather Package, Privacy Glass, Xenon Headlights
2007 328i 6-speed, Black Sapphire Metallic, Sport package, Black Leatherette, Heated Seats, iPod & USB Adapter
If God had intended man to walk or fly, he would not have made BMWs!