Remember, these are big trucks that take a lot of engine energy to get moving. The key to improving gas mileage is freeing up restrictive factory systems, like the intake and mufflers. Adding a K&N FIPK system in place of the factory airbox is a great modification that will improve mileage by allowing the engine to breath more freely. Adding an aftermarket muffler will do the same thing, but a mandrel-bent complete exhaust system will be even more beneficial by reducing not only flow restriciton from the stock mufflers, but smoothing the path that the exhaust gas has to follow. Gibson makes wonderful products, I use them exclusively on my trucks after trying out several other brands. I'd stay away from aftermarket headers if I were you. Exhaust companies kind of have a little advertising snafu going... you can free up horsepower AND mileage in most cases, but only to a certain extent; after that, its one or the other. Making horsepower requires two things, air and fuel. At a certain point you need to have more fuel to take advantage of increased airflow... not exactly good for gas mileage, is it? Adding aftermarket headers is a very time consuming process (even with Gibson headers, which are engineered wonderfully), but I wouldn't bother unless you are running forced induction of some sort; the factory manifolds are more than adequate for bolt-on mods. By the way, the vortex-style throttle body spacers are crap, don't waste your money. You can't "swirl" the intake charge - any "spinning" of the air created by those things is completely lost as soon as you get to the first runner in the intake manifold. Just my $.02.