For the question about adding another lower air dam on a stock height truck, I dont think that would help mpg since you would be increasing the frontal surface area of the truck. The stock front air dam, looking at it from the front of the truck, covers all of the front suspension and is the lowest point on the truck, and allows some air to still go through the 10" or so gap below the truck. The newer trucks have the front end kinda cone shaped compared to the GMT400s, with the headlights sweeping towards the fenders, and the bumper and air dams helping the air move AROUND the truck, our trucks are too box shaped to push air around it, the air goes over or under, so dropping the air dam farther would not, in my guess, help much. At least if it does, the amount of time and money you would spend doing that mod would never pay for itself in fuel.
For the post from HP Custom Paint, I had an experience to kinda go with your post. In college I worked on a senior design project called ECOCar, if anyone has heard of it. It was a competition for some of the top engineering schools put on by GM and the Department Of Energy to design a hybrid out of a Saturn VUE. GM was heavily vested in the project because they were basically getting free R&D from 15 colleges, this was before the bailout. We went up to Detroit and had an engineer meet with our group and he went over some really interesting aerodymanic research they had done, specifically on the C6 corvette vs the C6 Z06, and the tahoe/yukon hybrids vs regular tahoes. On the low pressure zone behind the vehicle, he showed how yes, there is a suction as it moves through the air, but they put pressure sensors on the back of a tahoe and found that right at the tailgate there were some high pressure areas PUSHING the vehicle forward. He said that designs like the 00s taurus' where they cone shaped out the back of the vehicle did not take advantage of this. If you look at the Tahoe/Yukon hybrids, look at the rear pillar next to the tailgate/hatch glass, they squared it off, instead of rounding it, and they found this helped create a high pressure zone pushing the rear of the vehicle, creating that bubble. Also notice the front of the trucks is low, with the air dams designed to move the air around the sides, then it hits the rear squared off area and the air is held in a bubble behind the vehicle. The majority of the air slides around the bubble, but the bubble itself is pushing the truck with high pressure. Every cell in my brain said "theres no way that is true, something is wrong here" but he had pictures and real data of red areas on the back of the truck that showed high pressure, and green and blue as low pressure, all from wind tunnel testing they did. Then you look at new pickups, with the little I guess wing? or extension whatever on the tailgates just like an inch or 2, that juts out rearward from the tailgate, kinda like a drag car where you put the flat wing out back on the car, to hold the air under that wing and create a high pressure bubble. The bubble itself is creating a high pressure PUSH on the car, with the rest of the bubble acting like the divots on a golf ball, air slides better over air than it does over anything else. Golf balls have divots because back in the day they found that dented balls went farther than new undented balls, and they found that air gets trapped in the divots, and creates little bubbles that the other air can slide easily over. Ive always wondered if you could cut the top of a new tailgate, or get the plastic piece from a new tailgate and put it on our tailgates, if that would create the bubble and help push us down the road, rather than us getting sucked backwards. There is so much that goes into aerodynamics and so much advancements in tools that engineers have now that they didnt have in the mid 80s when these trucks were designed. Look at the 70s front end vs the 80s front end, the 80s front end was DESIGNED because it was more aerodynamic, which is hard to believe since they both are boxes, but I guess making the front perpendicular with the road, instead of the top being higher and sloping down and in toward the bumper, pushed more air over the top and less under the truck, which is probably where the advantage came from. My buddy is still finishing his doctorate in engineering right now, hes doing diesel emissions research, and he is a huge gearhead, but he and I had a conversation about how now he has so much more of an appreciation for the engineering that goes into the stock trucks, and how he cringes anytime anyone modifies the tunes on the diesels that he works so hard to keep emissions down on. Same goes for aerodynamics of a truck, they are designed for all advantages of that truck sitting the way it does off the showroom floor, just raising or lowering the front end throws all of the deisgn engineers research out the window. Like the mythbusters and consumer reports tailgate tests, the engineers spent weeks, months, years designing the aerodynamics of that truck with the tailgate up, dropping that tailgate or putting on a tonneau cover throws all of that out the window. Like the little wing on the new truck tailgates, that is there for a reason. On our trucks, less time was taken on aerodynamics, so tailgate up or down could be positive, but there wasnt enough tools available to the engineers when they designed the GMT400s.
I have all of my college notes and books in a box that I need to break out, I remember in college we went over deriving the equation for how much hp you need to move a vehicle down the road at a certain speed. I had my 79 K10 at the time, and wanted to tow my 94 Z28 home to CT. I was using a Cd of 0.75 since thats about what big rigs are and my truck was no better than a big rig, the weight of the truck, trailer, and car at 9000lbs, the frontal surface area of my truck, and whatever else and it came to like 150hp to move it at 65mph. I never got to test this, as I doubted my truck was making 150hp anymore, so I just drove the car, but increasing the speed increased the needed hp exponentially. Which is why it may take 500hp to get to 200mph, but it takes 1000hp to get to 250mph. Moral of the story, keeping your speed down, is the easiest way to get the best mpg.
