2012 X5 with N55 and 8 speed is very economic. My N54 coupe ate a little more then my X5D as well. My business partner owns 2011 X5 35i gas and his consumption is barely noticeable over my D plus much more pleasant to drive then the clanking and lazy D.
Retired: 2005 N52 E90 330i Sport 6AT
Retired: 2007 N54 E92 335i Sport 6AT
Current: 2012 M57 E70 X5 xDrive35D