In the 300D 2.5 Turbo, the engine is turning about 3200 rpm at 130km/h (81mph). It seems that that model has quite a different gear ratio than your 300CD. 118km/h (73mph) is my normal cruising speed for the highways here to not get pulled over and then the RPMs are at about 2750 rpm.

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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)