heh I've been on teh bike for a few days and done some testing( I can 'feel' when I'm in teh slipstream better)
behind a transit, turbulence starts about 25-30 feet behind teh van. closer than 25ft and I was definately in the quiet zone and could roll off the throttle and still maintain teh same speed.
behind a car( all except MPVs) there was negilgable slipstream up to 10 foot, i didn't want to go closer than that
behind a coach the 'bad zone' was about 35-40 foot back, anywhere closer was definately quiet and clean air.