I average just under 30 in the Jag, it has been as high as 50 on a cruise down to Cornwall and as low as 14 on a hoon in Wales with my mates that very nearly set fire to my front discs and calipers
Not sure you can compare your Beemer and a truck, I have had a couple of diesels, my Leon and my Jag and from talking to guys on forums I have got about 50% better economy than if I had the petrol equivalent, and not lost out on performance too much either. Unless you count top end power over 80/90mph which you don't get to enjoy too much on the road, mid range grunt is epic, I have to admit on a long run I quite often overtake a load of cars and then let them all catch up just so I can do it again
chav