My 87 190DT gets 28 mpg while driving 80% highway @75 mph and using a/c 1/2 the time. The last tank got 30.2 mpg without the a/c. I have yet to test the mpg while driving 100% hwy. FYI, it takes ~1.5 psi of boost to run the a/c at 70 mph.

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1987 190DT, clackity clack
90 Mustang track car
95 Lightning

I've kept track of my '83 300SD on a spreadsheet for the 40,000 miles that I've owned it. My overall average has been 24.32 mpg. Highest was 27.23, and lowest 21.82. It probably could have been a little better, because most of those miles are in colder weather. I commute on my motorcycle most of the time when it is above 30-40 deg.

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Mike
1983 Mercedes-Benz 300SD 270K
1960 Ford F250 62K
1990 Mazda Miata 156k

The thing is Bob, it's not that I'm lazy, it's that I just don't care.

Finally drove almost 400 100% highway miles, average of ~70 mph. Got 33.1 mpg. I was hoping to get 35 mpg. Maybe with summer blend it will do it.

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1987 190DT, clackity clack
90 Mustang track car
95 Lightning

My OM603 1987 300 SDL gets 25mpg in town driving as fast as conditions allow. I get 29 freeway with cruise set at 75.

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1986 300 SDL - rolling parts car and test bed.
1987 300 SDL - semi daily driver.
1977 U1000 Unimog
2007 Ram 2500 6.7l