On the tailgate up/down thing, I agree that it depends on the truck. It worked on my '91 Ranger but hasn't worked on my stovebolt or some of the other trucks I've driven. If the tailgate is in the right spot it makes a high pressure bubble in the bed that makes air slip over top and out the back. If its not it just acts as a spoiler and creates a lot of turbulance which wastes gas.
The truck that the Mythbusters used to test was a new extended cab F- truck which was probably designed with aerodynamics in mind.
