The biggest advantage to drafting is seen by being very close to the rear bumper of the vehicle that you're following. As most posters above have already shown, this is incredibly dangerous. If you are within about 4 vehicle lengths you will achieve some fairly significant advantage so staying 3 of your car lengths back will benefit you, but if the truck brakes suddenly you'll be lucky to have enough time to duck in order to avoid the rear bumper as it comes through your windshield.
Another danger to following big rigs closely is your inability to see the road ahead of you, for sighting off ramps, traffic issues, road construction, etc. The danger versus return on this just isn't worth it IMHO.
>...inability to see the road ahead of you, for sighting off ramps, traffic issues, road construction...
Good point there - an important factor in economic driving is to have enough forward view to be able to react to issues well in advance (allowing you to slow down by lifting off the accelerator & hence using the zero-fuel underrun, rather than using the brake, for example).
Following closer than 2 seconds behind a tractor-trailer will also increase the possibility of rock chips on the windshield and sandblasting the paint on the car's nose. It makes the fuel savings seem minor.
As a previous trucker, I hated it when a car, minivan, etc would do this to me. Can't see them and I would usually slow down enough to make the guy pass me. If I blew a tire it may well end up going through their windshield and killing. NOT a safe driving practice - period.
Thanks, everyone. I had been following about 3 to 4 seconds behind trucks lately, or a car length for every 10 MPH, while making sure that I could see the truck's mirrors. So far, I think I've added 1 to 3 MPG. Not really enough to continue this experiment, but enough to satisfy my curiosity.
The consensus above indicates that drafting on larger vehicles such as big rigs saves a negligible amount of fuel. And aside from the safety issues already mentioned, there is also the long-term damage to your car's finish and windshield from road particles kicked up not only by the 18 wheels but also the turbulence from the undercarriage.
Ask any motorcyclist why he would never draft on a big rig.