I'm, also, not very impressed with the fuel consumption of the Compass, but it is, so far, the only thing that I dislike about this vehicle.
I'm averaging 19 mpg, at best, doing city driving and highway at fast speeds (75-80 mph). However, I was able to get around 26 mpg when I was on the highway doing about 69 mph with the cruise control.
Never been able to do more than 400 km with a full tank (250 miles). When I fill it up, it put 51 liters in it (the maximum).
I definitely believe, now, that the Compass's computer adjusts itself to your driving habits. I had a better fuel consumption in the beginning but the engine and the transmission wasn't as aggressive as now. I'm a very aggressive driver and that means one thing, very high fuel consumption and the computer now works accordingly, I believe.
I had pretty much the same story with 2 previous cars, a Chevrolet Venture (advertised as 11 liters per 100km, I was doing 15) and my other car, a Toyota Corolla. Both were eating the gas much faster than advertised and that was because of my driving habits and I know for a fact that the computer in my Corolla was adjusting itself to my driving habits, I asked Toyota and they said so. My guess is, it is the same for the Jeep.
That might also explains why other people have the results the other way around. Their driving habits are much softer than the "preset" standards in the truck so the computer adjusts itself (after a few thousand miles) accordingly.