I have a 2005 X5 3.0D auto with the 218bhp engine and most of my driving is on motorways (M6 J1 to J19) - on a good run I typically get around 31-34mpg, sometimes 37 - but that's rare.
Around town it's nearer 25mpg, sometimes 27. I reset the trip every 3,000 miles and it normally gives a 'long-average' over that distance of around 29.7 including all driving types. I'm not a fast driver, but I do drive at the speed limit(s).
Hope this might help.
It's not a truck, it's an SAV