OEM spec from MB is 33 MPG highway per MB documentation openly available.

No ethanol imposed on diesel, so no mileage loss besides out of spec, 30yo stuff.

Whether anyone ever got that is another story.

But I have gotten well over 500 miles in a tank in a w123, but not 600.

Ill bet economic point on this kind of a body style is 35-55 MPH.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)