For an additive, go to any Walmart and buy Power Service diesel additive. You'll see 2 different colored bottles, gray and white. For winter use you want the white bottle. The other factor in the low fuel economy is the very cold temps. This has been one of the coldest winters in a very long time. So I gather we are just use to warmer temps/better fuel economy. I just wish the temps would go up a bit so that I can change out my thermostat.

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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