I've noticed mine doesn't seem to change much regardless of whether its highway driving, town or even towing my boat. I was worried about a trip i went on last year that i would be touch and go if i had enough fuel. I thought the 4wding part of the trip would have used heaps more but i still averaged mid 11s for the trip.
On a side note, i did drive to brisbane from townsville last year in my brother in-laws 25yd D22. The tub was as full as it gets, plus 3 of us and a dog, the fuel use was around the same as my zd30 if not a little worse, coming home with an empty tub it got back into the high 9s. So it seems a load affects the 2.5 a lot more than it does the 3.