Perhaps, if you get close enough to the truck in front of you, you can attached some kind of tow rope between the vehicle you are drafting and your car - you could then drive for FREE!

 

If you get close enough behind a semi going at freeway speeds it can suck you in. The air currents curl in behind it, and if you get in front of the curve there it is.
In that case there is no free lunch and if you are that close it's the poor trucking company who is paying a portion of your own fuel bill.

I've been close enough behind a truck for it to disturb a terrible head wind (Following at 100-300 feet) a few times....but I even avoid that for incoming road debris.

I don't close-in draft and still get mid to upper 60's MPG, last tank came in at over 73MPG and 935 miles. But I don't go 70 either, I find another way.

I made this post to show that close-in draft isn't the holy grail of MPG, and that there are other more safe alternatives.

 

Too bad we don't have a truck driver here. I've sat in the cab of an 18-wheeler during a safety event here at work. They had cars positioned all around it so you can pick them out in the mirrors. Then they walk you around and show you all the cars you didn't see in the mirror. The point was to show you exactly what truck drivers can - and more importantly cannot - see. After that experience, you won't see me driving 40' off the bumper of a semi who has no idea I'm there.