baseline
If you have a really good baseline on the car,and after modifying you experience a change in fuel economy,you can calculate your new Cd through the back door.
At 55-mph,a 1% improvement in mpg equates to a 2% drag reduction.
At 70-mph,a 10% drag reduction will net you a 6% improvement in mpg.( apologize for mixed units,don't have calculator with me ).
Hope that helps.
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