what kind of gas mileage do mid 80's diesel mercedes get

1984-1985 5 cylinder Turbo 300D 29-31 Highway
1981-1985 5 Cylinder Turbo 300SD 27-30 highway (some claim more)
1987 300D Turbo 6 cylinder 29-32 highway
1986-1987 300 SDL 6 Cylinder 28-31 highway
190 Series diesels Avoid the 4 cylinder, 5 cylinder okay. Probably 34 mpg tops highway on it. These r all educated guesses talking to people who own them & owning a 6 cylinder diesel. Hope this helps somewhat, l tried to give a highway range. It depends how fast one drives as well.

My w126 om617 gets 34 hwy with 50% wvo mixed in.The veggie makes less friction.

1980 and earlier 300d (non turbo) will get 26 mpg consistently.
1981-1985 300d (turbo) will get a bit more.27
S class diesels from that era will get about 24, no matter what others here tell you.
240D is will get around 30, up to 35
190D is will get around 32-35, up to 40, regardless of engine
'87 300d is with the 6 cylinder will get around 29-30, if you find a healthy one