You are getting most of the extra MPG because you can see your driving habits. A tuner will not net you 4 MPG... I'm not going to argue the fact, but it simply won't.

 

My driving habits haven't changed one bit and I don't use the MPG parameter on the EVO. A huge factor is the ability to change the speed that the torque converter is locked at. I make the same 150 km trip from Calgary to Red Deer and back at least twice a week and before I had the tuner I religiously got 15 ~ 15.5 L/100km, now, driving the exact same way, I get 12.0 L/100km.

I was extremely skeptical about the fuel economy gains as well, but the thing works... and to reiterate, I have not changed my driving habits at all.