Count me in too in this silly question cause I was also going to ask the same question after reading it. How you tune a diesel engine after every 1700 to 3500kms?
However, a friend of mine used to tell me the fuel average of his 2.0D SE 2006 comming around 19-20Km/litre without AC which I didn't believe cause i once had a diesel corolla 86 with 2C engine and it never went above 13km/Litre.
Recently, somehow unwillingly I bought a 2.0D Saloon 2006 and I was really astonished when I checked its average on a long route. It covered 245Kms in 13.75 Litre diesel (approx Rs 1000/-) with AC. It calculates to 17.81 Km/Litre. I am neither a light foot driver nor I did anything specific to conserve fuel.
Journey was from Mirpur A.K. to Abbottabad so it was partially G.T. Road then motorway and then some hilly area.
This fuel consumption is simply amazing for me. I used Civic 02 Exi on CNG which I sold a couple of months back and that car consumed approx Rs 1,000 CNG on one side with AC. Then I had a Suzuki Mehran which consumed approx Rs 900 petrol without AC on one side trip.
So I am very happy with that. I have a powerfull car with a lot of luggage space, no CNG cylinder and queues on CNG pumps with same fuel cost as of CNG.