I thought it was the rpm highest gear that you got the best mileage. And then people LIED about where their sweet spot was so that they could justify driving fast.:B
My Phaeton seems to hold 6th gear down to 44 mph in economy, but I have no idea what the mileage is there but I imagine pretty good as wind resistance eats a LOT of HP.
I drive 61-63 mph when I am in no hurry which I wish was all the time. When I am speeding I drive 65 mph if faced with constraints like long days mileage, etc.
I like to know my mpg, but I like driving at a speed that will make for low stress travel not too fast and not excessively slow. But slow enough most traffic is passing my rig.