mythbusters found out it can increase fuel efficiency with 40% at 10 feet behind the truck http://mythbustersresults.com/episode80
Drafting behind a big rig will improve your car’s fuel efficiency.
To test this myth, the build team procured a car, a big rig, and a
device that could measure a car’s fuel efficiency. They then drove the
car behind a moving big rig at various distances ranging from 100 to 2
feet and measured the amount of fuel the car consumed. The Build Team
discovered that the closer the car was to the big rig, the less drag
is produced, thus the more fuel saved. At just ten feet, the car
managed to increase its fuel efficiency by 40%. Drafting at two feet
was slightly lower than the ten foot distance, mainly because Grant
had to keep working the car pedal to maintain distance from the truck.
However, that did not dispute the fact that drafting actually can
increase your car’s fuel efficiency. However, the Build Team has
warned that drafting is incredibly dangerous because the truck driver
may not able to see you and you may not be able to react in time if
the truck were to make a sudden stop.
the only video I was able to find was a 240p here you can see the full table http://www.youtube.com/watch?v=lttgT1XZVvE&feature=player_detailpage#t=534s
you can see that the economy starts at 100 feet with 11% saving over the control than as you get closer the economy increases up to 40% at 10 feet