In my experience most of them do about 12-16 MPG. Not really as good as you'd expect but the way they are driven is a BIG part of the mileage. I've seen claims as high as 24 but no one believes that.
On the other hand I have a new International TerraStar with the V8 diesel and Allison AT and it only gets 5.5-6 MPG @ 65 MPH. Just awful mileage. I hate this thing. So it all depends on your perspective and expectations I guess as to what is good and what isn't good mileage.
