I made the exact same trip this holiday season. However it was only me and my wife, but with a lot of luggage and presents (back seats folded down to fit everything). We got a little bit better mpg that you, but not by much, and I was going about the same speeds. And yes, the X is VERY comfy.

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*2010 X5d - Space Gray/Black Nevada/Dark Bamboo|ZPP|ZAP|ZCW|ZTP|ZRC|322|328|386|4NC|4UB|655 |6FL
~Ordered 8/27; Production began 9/28 at 12:20pm; Completed 10/7; Delivered 11/4!~
*2008 535i Black/Black - Dinan Exhaust & S2 Software
*1995 318ti Clubsport - Hellrot Red (autocross/fun car!)
Past: 2005 X3 3.0i; 2009 335xi