I read coasting in neutral uses more fuel then just coasting in gear. Supposedly if you coast in gear above a certain RPMs the injectors cut off until they reach a certain point where the IACV and the injectors open up to keep the engine running.
I seem to feel a slight "blip" at ~1700 RPMs. Then I down shift and feel it again. Again I haven't noticed any improvement doing this.
BTW. I don't draft semis normally, I just tried it once on a long trip. Typically I can't stand tailgators, especially when I am in the right lane or already stuck behind someone else. I either slow down, make my brake lights flash constantly, give them a wiper shower (if its a nice clean car), and maybe an abrupt tire chirping downshift for the worst of the worst.
I also carry a camera for those that are too special to read residential speed limit signs or right turn only signs (drive straight thru instead). It is awesome when they catch me taking their picture and look at me like "what did I do"? If only I could report them to their insurance company with some videos....