I get 20.5-21.2mpg everywhere, city or freeway, every time. Maybe 21.8mpg in the winter w/o A/C on. Have for over a decade now. The one time it dipped to 15-16mpg was when my coil failed, so I knew something was wrong right away.

I have Genuine MB fan clutch thats only about a year old. Does not stop with newspaper. When my original clutch died, I replaced both Fan Clutch, bearing bracket and tensionr with Genuine MB pieces.
I also wonder if the 79C thermostat option would work better for us in HOT climates, because it will open earlier, and open wider the hotter it gets. It should theoretically do the same as you are trying to replicate with the other thermostat.

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k