You don't *need* to be as close as two van lengths to get a good draft.
Depending on the vehicle you're following, the winds, speeds, etc.  you
should be able to feel when you get a "pull" at various distances.  From two
to about 15 lengths.  You see big trucks drafting each other, all the time
on the interstates..
  Just for information to extrapolate from, the aerodynamic figure given for
riding in the draft while on a bicycle is 20%.  The front guy uses 20% more
energy to maintain speed while all the riders in his (or other's in a line)
can go along using his draft to save energy.   Now riding a bicycle behind a
truck...you have to apply the brakes to keep from being sucked into the
truck's rear bumper..don't ask how I know this...hee hee.
  Don Hanson