[pciley]

Sorry Phoebisis, I have to disagree.. I believe a light soft tonneau cover will get rid of some of the wind resistance from the natural air flow over the cab and down into the bed that is normally blocked by the tailgate. Less resistance there. Quarter mile times were better with the tail gate DOWN rather than up, too.
I also get better milage with my CAI and Cat-back installed.
Any time you make an engine more efficient, you can expect to get a mixture of increased power output and decreased fuel consumption. That's pretty much the definition of efficiency on gasoline engines.