I know nothing of aerodynamics , but when I did my tuft testing, I had the chase car at a normal distance and passed me as my car was driving in the other lane.
There was no interference drag that I could see.
http://ecomodder.com/forum/showthrea...ack-21527.html
No other traffic was present at the time of my test ( perhaps one or two that passed by in the left lane .
)
Of course no interference drag will be present from the tailing vehicle, and I would guess the only time you have to worry about the interference drag is as the chase vehicle approaches the front fender of the car being tested.