I drive 81 miles a day in my SD with the cruise set to around 70 in the morning, and around 73 in the evening on my way home. It runs smooth and happy at those speeds. I have the 3.07 diff. I've been averaging about 26-28mpg. Due to where I live and where my workplace is, I only have to drive on about 2 miles of city roads a day, the other 77.5 miles is 55-70mph freeway. The one place the diesels really win out is efficiency in a head wind. Even when its real windy I still get about 23-24mpg.....but our 300E will drop from 24 to 17mpg if you do a long drive into a strong head wind.

As far as max efficiency, my SD maxes out around 30.5mpg or so if you go 50mph and drive like a granny. The 300E has managed 27mpg before after a long 55mph drive.

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