I generally run a bit over that 70-75 mph on the Interstate.....just a tad over 3,000 rpm.... Locally with these winding hilly roads in WV, I never get a long run at speeds around 65 mph.....

SB

Also of Note: According to the book MB book Technical Data Passenger Cars, April 1985 edition.....an '85 300D should get its best fuel mileage at 120 km/h or 75 mph.......

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG