Hello, I too have experienced this dramatic improvement in fuel mileage at times, always when traveling on a long trip when there is a moderate amount of cars on the road driving a consistant pace w/ little to no braking for traffic. I was so baffled one time that I consulted some aviation weather sites I frequent and got ground observations along my route and determined that I have had pretty good tail-wind components at times when I experienced the best mileage. The drafting effect plays a role as well, so the moderate amount of cars on the road played a factor to help the aerodynamic pig fight through the air at 70-90 mph. I just experimented with some 2 cycle oil in the diesel fuel, 14 ounces for the tank based on another post at another site, and it did run a little cooler and quieter, my fuel ecomony was 24.7 w/ variable winds and town driving. I wish I didn't have the 1985 300DT , 150k California model in almost pristine shape w/ the new trap oxidizerb just put in a few months ago based on recall, but I convince myself it's better for the environment, blah, blah... I badly want a 1984-1987 190D or 1986-1987 300 SDl; anyone want to trade?