I use Fuelly to see my km/l so I am very interested in fuel consumption.
When my mother bought her first bakkie, a 2003 K300 TD, it had a consistent 11.5km/l. It was very good because it was measured per tank from empty to full. In that 60L expedition (almost always town) it would obviously go over 11.5 and under. In my imagination I would be getting 13km/L somewhere in there and once I had the luck of doing so on a full refill. Sadly I have never been able to recreate the 13km/L tank and since getting the bakkie on 180 000km it has now come up to 250 000km last week and settled on 10km/ L.
I believe it is the injectors getting old.

There was a very interesting test on Topgear with a Pruis going around a track at max speed followed by a V8 M3. The Pruis was driven hard and the M3 only had to coast to keep up. It had lower consumption after the race.