Im sure the difference varies widely based on all kinds of factors. Craig, from what you said it sounds like youre talking about ULSD in huge power backup generators, thats ALOT different than an old diesel benz on the highway. its a much bigger engine running at constant rpms.

But i also seriously doubt that %12 lower mileage is possible when all we have is anecdotal evidence. I also get alot lower milage on the highways than i did a few years ago, but i think a more likely cause is me having to speed up and slow down all the time because of all the people driving slower because of the high fuel prices!

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350