[Cd]

BTW, speeds in the above test were approximately 40 mph, and the chase vehicle was a Saturn coupe.

If you haven't done the test yet, I'd like to suggest adding tufts at the C pillar and trailing on down to the edge of the trunk/boot.

Of course a visible color of yarn is preferred. When I did my test, I found that using super small diameter line ( "string "? ) seemed to give better results than actual yarn, since the yarn has " hairs" which stick to one another and cause drag from the yarn itself .

I used line that was perhaps 1/ 16 " or smaller, and no longer than 3 1/2 " - 4 "

Any longer and the line gets flutter which skews the results.

( I tested several colors for visibility as well as different lengths and materials - yarn and line.