[Big time]

I wonder if it has ever been tried.

Mounting the model over the roof of a car and measure its drag. You can even place smoke generator oulets in front of it.

Of course you will need very tall mounting brackets to separate the car model from the speeding car by several feet, so the flow distortion from the car won't disturb the model.

Maybe you would also need a extremely streamlined car (kind of solar racer) to minimize flow distortion?
Maybe you'll need to place the car model over a board parallel to the ground to simulate ground effect.

I know it sounds weird but has it been tried or proposed before?