[Angrybird 12]

Having driven my son's Silverado a few times and watched the AFM system working I know for a fact that
AFM engages only when you are in top gear and either holding a constant throttle position on have your foot off the throttle.
Anytime you are "Giving it gas" to either hold a constant speed even on the slightest incline or trying to accellerate the AFM will Disengage and you will be running on all cylindars.
In my opinion if you live it flat country you will actually see AFM help, but living in the mountians like I do or even in city driving it will not have very much effect on your MPG.