My manual 240D got about 16-18mpg when I first got her.
After much work for 3 years on my and my brother's part, it's now getting an average 25mpg.
And this is driving a very short trip to work, with longer trips throughout the day to various sites.
I've noticed several things that seem (to me) to make a difference, and I'm new to this really, so forgive any ignorance.
----valve adjustment- definitely brought up the mileage and she runs much better properly adjusted. There are lots of great posts on how to do this here, or you can pay a shop to do it.
----diesel #1=bad mileage. It may be great in the really really cold winter days, but diesel #2 seems to give me an increase average of 3mpg every time.
----driving. If I drive like a moron, I pay for it
----highway miles at 55-60mph on a flat run give me the best mileage
----as always, make sure all filters are clean, she gets proper maintenance and oil changes.
Good luck.

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Nicolette
1977 240D