On a long trip in very heavy traffic I figured that trying to pass a few cars would not benefit me in any way so I put on the cruise, opened the sunroof, turned on the stereo, leaned back and relaxed and drove at steady 90 km/h (55mph) at 100km/h limit and to my amazament got as low as 9 litres/100 km!!! that's about 31-32 mpg isn't it?
Soon as up came some hills it got up to 10 l/100 -which is pretty low, too, about 28-29 mpg I think - but driving steadily at flat terrain gave unbelieveably great economy for a heavy luxury sedan!
I average about 13-14 l/100 since I drive a lot on the highways but this was an all-time low for me!
got 25,5 l/100 when I was fooling around in an urban area :-)