You need to do 5,000 miles min before a modern diesel gives the best mpg. If I go on the trip computer I can get 125mpg in eco mode for about a minute.
actually VW are one of the better mpg computers they under estimate, when I first picked up the car with 10 miles on it the first 200 miles it reported 26mpg ha ha ha, so I filled it to the brim again and done a tank to tank and it was 32mpg so I believe the VW onboard computer under estimates a little (not saying it does for the other vag group cars — audi—skoda— and seat, but then again the speedo is out by about 4mph less on my tig so it could be that).