Found this today.
It's an exhaustive study of the effects of vehicles drafting each other at close range. An aero nerd's dream. (That would be me.)
Lots of pictures and data, pressure gradiants, surface flow visualizations,

etc. Most interesting is the back to back of the Lumina van which nets by far the highest drag reduction at 0 distance: .575 of baseline, average reduction in drag . Can you say aero trailer?
http://www.path.berkeley.edu/PATH/Pu.../PRR-95-35.pdf
Some images...
