Mario, My mileage has also plumeted in the last few weeks. Though I do have problem with the thermostat, it seems to be stuck open. The engine is running 20 deg. cooler and the air through the vents on max is luke warm. What temp. is your engine running at? I also believe aside from that, it's just too darn cold out. I regularly use Power Service winter additive. Hopefully when I get a chance to change out the thermostat, my mileage will improve. Right now with diesel costing about $1.75 a gallon, I need all the help I can get.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member