Here is a chart that shows what a 300sd might do. The 300D is a bit smaller but not as smooth aero wise. It might be similar.
According to an engineer friend of mine it means that it takes 20 kw of power at 2500 rpm and it is burning 300 grams of fuel per kw. or 6000 grams of fuel when running 2500 rpm. Say that is 55 mph (It is on my car.).
That means you can go 55 miles on 6kg of fuel. He told me that diesel weighs about .8kg per liter. In other words your are using 7.5liters of fuel to run 55 mph. Or about 27 mpg. If you go 3000 rpm you get about the same efficiency but you are needing 30kw of power.
He went on to say that by looking at the graph the car is geared too low for economy. I understand fairly well what he was saying but I looked at the graph for some time.
I am one of those who generally likes to conserve fuel.