This is a case where empirical knowledge needs more weight than physics calculations. The people at Ecomodder.com (
https://ecomodder.com/) study the practical effects of aerodynamic drag ad nauseam. One of their heroes modified a Honda Civic until he was able to get 50 MPG at 90 MPH, and 90 MPG at more reasonable speeds. He built the car because he drove a lot of miles, mostly at high speed, and wanted good gas mileage. The story is at:
https://aerocivic.com/ and also at:
https://ecomodder.com/forum/showthread.php/aerocivic-how-drop-your-cd-0-31-0-a-290.html.
The short version is that gas mileage improvement in the real world is about half the decrease in aerodynamic drag. A 10% reduction in drag typically yields about a 5% improvement in gas mileage. That will vary depending on many variables other than aerodynamic drag. Also, reductions in aerodynamic drag require other changes in order to realize the full benefit.
The best book on automobile aerodynamics is the book by Hucho:
https://www.amazon.com/dp/1483108414/?tag=pfamazon01-20. Highly recommended if you are interested in the research on the subject. If you want to learn practical techniques on how to modify your car to reduce drag, the best book that I know of is the book by Julian Edgar:
https://www.amazon.com/dp/1787112837/?tag=pfamazon01-20. Highly recommended if you want to modify your car and do it right.
One person, who shall not be named because that would be bragging, made a number of modifications and documented the effects on gas mileage. Link:
https://ecomodder.com/forum/showthread.php/modding-06-gmc-canyon-17070.html. In that case, the gas mileage of a 2006 GMC Canyon crew cab truck was improved from 21 MPG in winter and 27 MPG in summer to 32 MPG in winter and 38 MPG in summer.