The best I've ever managed in the NA 850 5 speed wagon driving back from Seattle to California was 32mpg. We averaged about 70mph with stints up to about 90mph on the down hills through the mountains in neutral or in gear depending on the circumstance. If I do a constant 55mph on flat ground, the trip computer shows 39mpg. I don't think I could ever drive that slow for any length of time so its purely theoretical.
The 850 wagon and V70 have a CD of .31 which is excellent considering the squared off styling. Volvo actually put a lot of wind tunnel time into these cars to fix small details like smooth out all the transitions from the roof to the sides and to the back. It looks boxy but its actually a very clean design. As ALS would tell you, there is plenty of room for improvement on these cars. He gave me the idea for the underside panels and I've been very happy with it thus far.
good luck
Justin