MB Engine design
 

It is my belief that the MB 6's and 8's were designed to cruise at autobahn speeds at 2600-2800 RPM's in a power band that was designed to give the max MPG/KPL possible (given the limitations of the FI system and engine component weights).

Example: My 300E M104 gets 32 to 30.5 MPG at 75 MPH when on cruise from NYC to Charlottesville-more than my Volvo 240 2.3 I4 less 500 lbs of chassis weight-same road, same gas, 500 lbs heavier but still beat my 240 by 4 to 6 MPG.

If you drive it in a consistent pattern, moderate accelleration, go to cruise when you get to 55 and use the cruise accelleration controller to reach 75 and then just hang out, your MPG/KPL will be consistentky in the top end of the MPG/KPL curve.

I did a project for eng class on drag associated with suinroofs and open windows at 75 MPH and found the loss of MPG/KPL to be less than 1 MPG/1.4KPL over 600 miles (longest I could drive without stopping for gas/human system fluid drain).

go figure...

I bet the gas tank also gets topped off at different levels
each fill up. One more random factor , tailwind etc.

Good point. I was down to about 2 gallons left in the tank when I filled up and the extra 18 gallons is a significant weight. It wouldn't surpirse me if I got 32-34 mpg on the last half of the tank vs. 26-28 on the first half for an average of 30. That number I gave for my worst mileage (15mpg) was actually just based on half a tank before I filled up again. The extra poundage in fuel probably makes a bigger difference in the city.

25.55 mpg is very good.

My personal record for my 2000 E430 is 26.1 mpg, which I got driving from L.A. to Las Vegas, with average speeds of about 80 mph.