The laminar airflow across a vehicle turns to turbulent as it leaves the 
rear.  Some of that turbulent flow will be reduced if there is another 
vehicle close behind to "grab" the airflow.  Laminar flow creates lift, 
and turbulent flow creates drag.  

Last winter we had quite a bit of snow in western Washington.  I 
frequently travel by car from Seattle to Bellingham, and on one occassion 
saw an accident in the snow on Interstate 5 just north of Seattle where a 
Vanagon had run into the back of a cement mixer.  The mixer's "funnel" 
was in the back seat of the van.  

Also, about tires, one time in southern California on the 210 freeway in 
the middle of the night I ran into a tire which had just blown off of a 
semi truck's wheel.  I was driving my Scirocco, and the tire and frontal 
spoiler (air dam) both bounced and dragged under my car until I could pull 
over.  The spoiler broke in half, and in a few other places too.  At 
least I wasn't right behind the truck when it blew, or it could have come 
right at my window.

Erik Peterson
Vehicle Research Institute