I was drafting a little last week on the interstate. It seemed like I could get a pretty good draft off a tall semi with the standard box trailer from about 4.5 car lengths back. I felt pretty safe back there and was surprised at how much it was helping.
I say I was getting a good draft just because of the decrease in wind noise and the reduction in throttle I had to give to maintain 70 mph. I also noticed a sweet spot that is behind and diagonal to the semi trailer (in the next lane over).
Yeah, definitely. Gas savings from just driving slower and further savings by getting sucked in behind the truck. How close do you draft? You know, I've never seen a truck stop short. How unsafe is it really to draft close behind trucks? I mean, what are the odds? Prolly worth the risk. Worst that happens is you total your car. Not likely you'd be seriously injured. With saying that, I don't feel comfortable enough to tailgate close to trucks.
I personally don't think it's that dangerous, but you have to be more aware and act quickly if anything happens. I follow pretty close, up to about 3 vx's length from the truck, but usually I stay around 5-6 car lenghts back because trucks don't like it. I find that if they keep changing lanes they are telling you to back off.
Drafting definately adds about 8-10mpg to my avg FE.