Moblie Wind Tunnel (sort of)
 

Thought of this the other day when it was raining.

Instead of taping tufts all over the car, I think there may be an easier way.

No need for a camera or a chase vehicle to check results.

The windshield washer. Just add some hose to the washer pump and a tape a nozzle anywhere you want to test the aero and spay while you drive pull over and check out the water pattern.

You could even make it more permanent. Put a small aquarium pump valve inline on the washer pump line. This would allow you to enable or disable it (under the hood) or run as many lines as you wanted.

Lots easier than explaining why you have yarn taped all over your car. Plus you could set it up to spray innocent passers by. :)

On the weekend, I saw a Chevy Cavalier with a unique hood ornament: a kitchen faucet. Seriously. With a coffee cup underneath, glued to the hood. I'm not sure if he had it connected to his washer pump or not. :)

But seriously - "drip" testing is another method some people will use to trace flow patterns. Dish soap drops placed in a row, then go out & drive.

I think you'll get more accurate info from tufts though...