Doug; you say you get 800km to 50L of diesel. Must be a VW jetta. The last tank got me 58.25 MPG commuting from home to work at the Petro Canada Refinery(I JUST WORK THERE,OK!)
My jetta is an '02. When I first got it it was hard to sqeeze in $28 worth of diesel. The other day I put in $60 worth! And that was before the recent price increase!
I have found that driving for mileage makes a huge difference in your mileage. My pickup went from 18 to 26 MPG and my motor cycle went from 56 to 71.
Keep tires inflated properly
avoid fast starts
accelerate slowly
accelerate on down side of a hill
don't accelerate on up side of a hill unless necessary
coast downhill
shut off engine at long stop lights
drive at or below speed limit
when coming up to a stop light, shift into nutral
use cruise control if you have it