I would say fuel prices played a part as well. I'm sure many people aren't running to 'convert' their cars or buy a diesel when gasoline is under $2.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD