BMW Z4 3.0i
i got 36mpg on a 180 mile motorway drive sat at 80mph
thats bad, i can get 40 at 90 :S
I think I'm quite an economical driver when I want to be, though admittedly it's kind of out of your hands anyway travelling at constant speed on open motorway. But I never achieved over 40mpg doing 90mph in my ph2 172. It was a little while ago now but I used to do shitloads of motorway driving in the middle of the night and my memory of the economy figures were approximately:
70mph - 45 mpg
80mph - 40 mpg
90mph - 35 mpg