It was a Mythbusters episode, and they concluded that you needed to be following the truck extremely closely (I believe within a couple of feet) to see a gain i

It was a Mythbusters episode, and they concluded that you needed to be following the truck extremely closely (I believe within a couple of feet) to see a gain in fuel efficiency. The driver was unable to keep that small distance consistent for a continuous period of time causing more speed variations, resulting in a loss of fuel efficiency.