I believe this has been discussed earlier but as I understand it the optimal MPG's is achieved where the engine is working just above the maximum torque in the highest gear which I guess is 2400RPM's for most of the MB cars. There are a lot of other factors that affect mileage the biggest other than the engine itself is the wind resistance which increases at the sqaure of the speed and not linear so you have a 4 times the wind resistance at 60mph than you do at 30mph. The diesels aren't designed to run an extremely high speeds either. The fuel's combustion rate is slower than gasoline and therefore it's efficiency diminishes as the speed increases. You can see the decrease in performance when looking at the horsepower and torque charts.
Of course the best way is to fill the car to the neck and drive it with the CC on a road trip and see what it gets for each leg at different speeds.
