see the drag (simple method for visualizing airflow patterns)
 

you can see the drag if you mix up lampblack with water. not too runny but too wet to dry fast. be ready to move. then paint about a inch or so dots on 1/2 the car. drive fast. the dots will follow the airstream and show smooth flow and turbulence. if your a cut and tape man you can see more or less were to fill in. i don't think it will matter on the street that much, because back in the day ,i was a kid reading about it, you could go to a beach speed week and see what yours could do on the harder sand with their clocks. after they got their fastest time some would cut and tape. from what i can remember the top would go up under 10 mph at about 130. and the cars back then were funny boxes. maybe someone who is doing it could go to the library and look up old car mags from the late 50's and early 60's. take a look at the lead sleds to. an idea to fix it on your car maybe, is to tape it and fill the back with spray foam. wax the car, so it will come out easily and glue it back with silicone glue. it will come off the car if you need to work on it, or sell the car. sand and bodo it to a smooth shape.. i don't think silicone will melt that kind of foam but you should try on a small patch first

Using wool tufts is also a good way to test if you have turbulent airflow in certain areas. It sounds easier to clean up too.

See here

I've read something along these lines as well DAN - except using dark food colouring in dish soap. (And the dish soap washes off in a nice lather when you're done the test. :) )

I suppose the most obvious benefit for doing "drip testing" is that you can do it by yourself and view the results when you've stopped, rather than taking a passenger along with a video camera (or an additional car, if you can't see the tufts in the area you're interested in studying).

^^ I've seen oil used too :)

the only problem with water, oil, etc. is that it's surface tension interferes... This makes it good for looking at bulk flow - but not so much for sporadic flow interference. Keep in mind, there is no easy method in real world testing (other than tunnel and smoke) that won't interfere :)

One thing about cars from the '50s and '60s is that their aerodynamics were terrible- similar to today's SUVs and pickups, if not worse. There was more room for easy improvements.