I'm averaging 19-20 mpg a tank driving in M5. This is mostly miles at 55 - 75 mph in hilly/curvy country. 6th gear is pretty much useless where I live I've concluded. I'll use it when on a Interstate type highway in relatively flat country, but it won't help me where I do my daily driving.
Somebody asked about "WHY RUN IN M5"?.... To prevent excessive oil consumption because of bad rings and pistons......ask a good mechanic or do a little google search. GM says 1 qt of oil every 1000 miles is NOT excessive. I disagree. Running in V4 causes this problem.