Next time try with out cruise control, even on my F10 five series it leaves a bit of throttle on going down hill....or even better get your wife to drive, my wife borrowed my 5 series with her buddies to do a road trip and got 870 kms, best I've ever got near to is 600km...go figure :-)

Got 1000km out of a 335d I had easy...

Cheers
stu