The economy tune goes to 4 cylinder mode sooner. There is no power gain with it. 87 tune has a small gain and Diablo tune (93 octane) is the good one, and the one that almost always gets the best economy. These trucks are grossly underpowered (and I mean GROSSLY. 400hp in a 5600lb SUV? Even worse, 300hp in a 6000lb pickup truck? Come on GM!!), so the tunes that make the most power help economy because the motor doesn't have to work as hard or rev as high.