I agree. 22 is about what I would expect. I measured every tank in my 300SD and the range was 19-27 ish. Similar weight but more aerodynamic than a 123 wagon.

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1979 240D, 4spd manual, Power Sunroof, manual windows, 147k miles, Pastel gray/Black MB Tex.
1991 300D 2.5 - Smokes like it's on Crack!