The difference between 29-30 mpg to 20-21 mpg is enormous. I wonder how did one not notice it during several months, that would be approximately 200 miles per tank filling; No need for a calculator.

I always reset the mile counter when filling up and keep a rough visual estimate on the mileage everytime my odometer needle reachs the 1/4, 1/2, 3/4, reserve marks. I don't have a computer telling fuel economy but 50% difference would be very noticeable to me.

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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE