LDMF

The Car: 2005 E Class 320 CDI with 160k on her, aftermarket 17" wheels with
Sumitomo HTR Z III (245/45ZR17 Y).

In the past week I drove from:
Pittsburgh to Savannah
Savannah to Sarasota
Sarasota to Helen
Helen to Pittsburgh

The car performed flawlessly. The mileage didn't seem to change much from the mountains with an average of 69 mph to the flats of Florida with an average of 57 mph. I used cruise control extensively. The average for any tank was 35.5 to a high of 36.8. Came in for the trip just over 36 mpg. That is with the wife and her 47 suitcases.

I didn't get the 40+ I read on here, but I was very happy. The only thing I did notice is that at highway speeds and brisk acceleration it does smoke a bit. But maybe that is normal.

rapidoxidation

I did a run back to the east coast last summer and had a day of burning about 2 gallons an hour at 80mph. Might've been a nice tailwind, might've been some freaky high energy diesel, but upon filling up I was confirmed to be an honest 40mpg at highway speeds for the day.
I love this car.

arto_wa

I have driven about 12,000 miles since buying 2008 W211 BlueTec in August and the average mileage for all of that was exactly 33 mpg (manual check).
That includes driving on highway, city, mountains, hot weather as well as way below freezing and snow with summer tires & winter tires.

I do see over 40 mpg often in the computer on a highway but that's what it takes to get average of 33

no gas!