Bonneville Landspeed Racing - Let's Try a Thing.

How the hell do you calculate that? Look into active shutter assemblies. Or mechanical in your situation.
There are formulas to figure it out, but from what Chrysler engineering docs say, the stock Charger was approximately 0.44, the Charger 500 in race trim was 0.375 and the Daytona was 0.28. Shutters? For the lower grill opening? The upper is only open as I ran out of sheet metal.

Vehicle aerodynamics