Hello,
the wind tunel is a very expensive method of measurement.
You can improve the quaity of your aerodynamic by wool threads on the surface.
For fine modifications, I made testdrives, then I changed the shape.
Example: modification at the wheelhouse.
The wheelhouse are very sensitive, so little changes can bring a other airflow.
Before you go in to the windtunel, make sure that you can modify the car in a short time, to compare the original car with the modifications.
A flow meter diagram can also help, to estimate the results.
Measure with open and closed air intake for the heat exchange.
More photos on flickr:
https://www.flickr.com/photos/ulrich-steinlechner/
Kind regards
Ulrich Steinlechner