Could not find a reference for the rear axle ratios for the cars, but I did see a quote from MB for the mileage in the city for a W123 200D, and it is 9.5 liters per 100 km, or a bit better than you are getting. So, it may be that your axle ratio has the engine working too hard to get the car moving. Chances are it will adversely affect the life of the clutch too. Good luck and sorry I could not get a little more definitive. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)