Swedish carmagazine Auto Motor & Sport did a test between four economical dieselcars. And drove them as long as possible on one tank of fuel, they had to have a average speed of 80 km/h or above. Winner was (the current 2.0 TDI) VW Passat Blumotion, it came fron Stockholm to Paris – 1954,7 km – on one tank of fuel! Wich means 3,4 l/100 km!
Source: Auto Motor und Sport, thanks to Jesper for the tip!