I can't drive slow enough to achieve over 40 and I know I never will but I do know when I use 91 over any lower grades I seem to get an additional 30-40 more miles a tank and that's with nearly all city driving. Still impressive was the 32mpg I got going over the Continental divide and back (700 miles) and was solidly over 70mph the whole trip and even paced an Audi at speeds over 110 for about 20 miles.
'12 Focus Red Candy SE Sport 5 spd
'99 Toyota 4Runner Sport
'11 Subaru WRX STI Hatch