Once again, I will quote from my english language owners manual that came with my 84 euro 300TD turbo.

According to MB a w123 300TD non-turbo, automatic transmission (of undisclosed year) should get:

21.38 mpg at 75 mph continuous speed

and

29.04 mpg at 56 mph continuous speed

and

24.00 mpg in Urban Driving

The turbo version is slightly better at highway speeds (21.56 and 29.40) and worse in the city (22.62).

Slow down there lead foot. Quit the jack rabbit starts, tailgating and accelerating until you need to brake. In short, stop driving like all the other idiots out there and you will get much better mileage.

There are things to check here too, it's not just driving habits although they do impact fuel economy.
1. Valve adjustment.
2. Air filter.
3. IP timing.
4. Compression
5. Restriction in Exhaust system.
6. Trans not shifting at proper RPM's and speed. (This is one of the easiest to notice.)
7. Type of fuel. In my experience, Biodiesel gets better fuel economy.
8. Type of engine oil. I also use Lucas Friction modifier. Just adding this makes my engine RPM go up by 200 and my oil pressure from steady at 2 to almost pegging 3 at idle.