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So, you want to modify the aerodynamics of your car? Great! These posts explain how to get started testing and measuring things.

Identify goals and make a plan
Measure, not guess (you're probably wrong anyway)
It's easy and cheap to measure pressures on your own car
Learn what yarn taped to your car can tell you about the airflow over it
Want to lower your car's drag? Here's how you can measure it
A problematic method for measuring drag--with my suggestions for improvement
Basic engineering practice to keep your calculations realistic