I just got about 32mpg on my last tank (13.6 km/L) driving 120 km/h most of the time which is about 2750 rpm for me. I don't ever do much better than that, only slightly.

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1991 MB 300D 2.5 Turbo (207000km) (now gone)
1993 Audi 100 CS Quattro (300000km)
1999 VW Jetta TDI (290000km)
2002 Audi A6 3.0 Quattro (150000km)
2005 MB E320 CDI (34000km)