Where's the best place to test aero? How about a tunnel?
 

The secret racing test tunnel no one wants to talk about

A one mile long abandoned tunnel. A testing environment most of us can only dream of...

Ummmm. The guys at Synergy Aircraft eschew wind tunnels because they invariably interfere with the actual results that happen in free air. For an airplane, it's a huge deal. For a car, it may not be as significant, but still far from a "dream set-up", unless you want to drive in tunnels all the time, in which case, this is awesome.

I wonder how the limited cross-sectional area of the tunnel impacts the testing? Probably no worse or no better than wind tunnel testing.

The tunnel tunnel testing looks like a good idea to me if one wants to eliminate winds from any direction from the tests, use the actual vehicle instead of a model, and doesn't have a fortune for wind tunnel time.

Aircraft don't have to worry about yaw forces/effects unless they're interfacing or about to interface with the ground.

I'd think if the tunnel were wide enough, there shouldn't be too much of an issue for cars, right?

I think it comes down to percentage of "blockage" i.e. vehicle frontal area vs tunnel cross sectional area. I'd imagine a vehicle- especially one that booking along like these race cars- would create a bow wave. The question is how significant is this wave and how does it impact the results?

A high-speed camera or video and a bag of flower now you can see if thir is a bow effect,and more important your airo shape.
Add a leaf blower to stir the flower into the air
Just an idea. I've used a sandstorm before. Park into the wind 30+mph wind recommended