Nickofoxford 3000rpm = just over peak HP (which I think is something like 2650 but I forget) so better mileage. Faster than ~65mph and you'll be using so much extra power to overcome wind resistance you'll lose mileage.
Best mileage is definately on the highway, longer runs at 55-65mph. Don't just say I'm wrong do some tests and prove it. I drove a 240D on a 45 mile each way commute for 2 years, when I could managed to keep my speed to 65 on the highway or less my mileage was much better. I never did significant tests at 55mph to see if there was an improvement there, there might be but I couldn't stand it long enough to find out.
The trick to high mileage is to drive long trips, short runs are what bicycles are for

Nicolette is right, you will always lose a couple mpgs from winter fuel. I also found I lost a couple mpgs from snowtires. I see that on my 190D also, I change back to summer tires and mpg goes up immediately.

Its interesting that clogged fuel filters hurt mpg but its true, so does a clogged tank vent. Both of my cars have at some point had clogged tank vents. The 240D is pretty easy to clean, easier than the 190D anyway.