I get the best mpg at about 70-73 level highway. The 3.0 seems to have maximum efficiency about 2500 rpm in my case. On the afformentioned trip, 2/3 of the trip is 70 speed limit but also with plentiful elevation changes.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)