Just my two pence worth.
When working out your fuel you will not get 3km per ltr. I have got a 4 series scanny, and run fairly light most of the time(under 30t gross), and generally run home empty, and I average 3km per ltr. When heavy (38t upwards) this drops to about 2km per ltr. This gives an average of 9 mpg when light and dropping to 6mpg when running heavy.
ps. My motor is the most economical motor on our fleet.