I've heard multiple times that people will put themselves behind trucks while driving on the highway and consume only half of the gas they would have used if they weren't behind them. It's probably related to that.
Bikers do this too BTW if you follow the sport, makes for interesting team type tactics.
There is some thought that automated road convoys of trucks following closely might generate fuel savings this way as well.
Funny, my current car does 24 mpg on the highway at the speeds I like. Half the efficiency and probably mostly due to me.