Hypermilers do this - pretty fun / a bit dangerous.

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.

Cyclists ("bikers" is an ambiguous term) enjoy a 30% reduction in required power output to move at typical race speeds when within a reasonable size peloton compared to riding out on their own.

It could also be because trucks drive efficiently with little speed variations, at speeds of high fuel efficiency, 80-90 km/h. Drag is actually much more complicated than that, the air being is turbulent.

May be a little, but classic hypermiling technique is to do that anyway. So the slipstream gains are real since absent the truck a hypermiler is going to do the same thing anyway. I never hit the heady heights of good hypermilers, but I had a car I could reliably do 23 km/l, 55 mpg in.

Funny, my current car does 24 mpg on the highway at the speeds I like. Half the efficiency and probably mostly due to me.

Semi under 12 tons can go up to 110km/h in France, and I think most of them drive around that speed.

I remember seeing either Mythbusters or TopGear episode where they tested this.

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.

It's called slipstream I believe.