Wooly Tuft
 

When a car is in a wind tunnel the aero properties (and drag) are examined with a nice clean flow of air hitting the car from the front.
If we are following a Hummer hoping to be suk't along by it perhaps we are not close enough and caught in that buffetting zone.
Surely this turbulent air would be cause us to lose FE , as it upsets the air flow around our cars body.
Maybe a 10 centimeter bit of wool tied to a windscreen wiper would act as a rough gauge of the wind turbulence ahead.

Back in the good old days...there were herds of Wooly Tufted Mammoths along the roads. You could stop at any time ...get some tufts...and do your testing. Now...you don't see so many. :confused: :D

Back in reality...I don't think you'll see a lot of gain unless you get past the buffeting...and then you are too close for comfort.