Or far more simply: just have a chase car and an empty road, its cheaper and easier
I did a few searches trying to find everything I could to get more data, but one interesting piece I found had to do with wheel skirts.
From Gassavers - basically it seems the pumping of air from the wheels combined with a wheel skirt created pressure that messed up the airflow and reduced MPGs. Without the spokes pumping pressure in there the situation might have been reversed. Perhaps step 1 is disc and step 2 is skirt?