850 glt 2.5 20v bad mpg

I personally think that driving style is the most important factor when considering mpg. I can only take my mpg from the trip computer. When in neutral coasting downhill (not something I do often) I get 99.9mpg, when accelerating reasonably hard in a low gear I can get below 10, cruising on the motorway 60-70mph I can acheive well over 40mpg. The average mpg currently shows 30.9 mpg, that's a mixture of town and motorway driving.
I don't drive my car hard very often, obviously with a T5 like the rest of you I do drive hard now and again.
My car is mainly used to ferry around my 4 month old daughter with a Whinie the Pooh window sun sticker!! My wife drives it frequently, I think tahat explains the scuffs on the alloys!
If you drive sensibly then you should achieve a good mpg (relative to a Volvo).