I didn't bother reading your links about fuel economy since i've got around 45k direct experience with cdi engines and there is now way in the world i'm going to buy your 31 mpg 'hope' based on the data you provide. Sorry, no sale here.

My glk diesel is returning 37-39 mpg highway mpg on a consistent basis and there is absolutely no doubt in my mind a c class diesel with the om651 engine would be in the high 40's at least.

I just filled up my 06 e class today and got 41.5 mpg on the last tank, mostly highway.

I'm betting the e class with the om651 will indeed make 45 mpg on the open road.

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98 Dodge-Cummins pickup (137K)
13 GLK250 (157k)
06 E320CDI (341K)
16 C300 (89K)
82 300GD Gelaendewagen (54K)