My 159 has around 120.000 kms (about 74k) , and I'm doing consistent 7l/100km averages (that's around 40 mpg). The trip computer is quite accurate, and, reseting trip A after every fill-up, it's giving me 600+ miles of fuel autonomy. And I really put my foot down, sometimes... All in all, we're talking about Alfas, right?
My previous S-Max, beeing heavier, and with the 1.8 125cv, would give similar figures, so, the 158 isn't all that economical, I'm afraid. But, to my understanding, is a good compromise.