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NACA and its various equivalent research groups studied this in conjunction with biplane wing arrangement, where both wings are smooth and streamlined. What is the optimum gap between wings? Too far apart and they lose the synergistic flap effect, where they produce more lift than the sum of their respective lifts. Too close and the interference drag increases, since the airflow beneath the upper wing conflicts with the flow over the lower wing.
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I know Lost Cause already responded to this - but I've got something to add...
Bi planes are a good example of parasitic interference - but not a good example of how that applies to cars. First, the ground isn't static as LostCause said - we're observing a car moving through air - not air moving over a car (ignoring wind). There is no boundary layer at the ground. In order for interference to occur - the boundary layer of the car must all the way down to the ground...
All that, plus interference increases as a function of v^2....
227mph airspeed.
70mph ground speed (max)... 100mph air speed (even that's fast --- I sure as hell won't be driving in constant gale force winds
)
Generally, it's a slippery slope to compare the aero features of a streamlined vehicle (plane) to the features of a bluff body vehicle (like most of our cars).
Now, I'm not saying interference drag isn't an issue - I'm saying I don't know.But honestly, I'm leaning more towards the insignificant range -- that is, you'd have to lower your car to impractical levels for it to have a measurable effect. I say this because world records are broken with super low and super high vehicle shapes