you get this figure from a projection of the car's forward facing surfaces onto a 2 dimensional plane, then integrating the area. Hard to do without a CAD drawing (and a computer), as any photograph would introduce "perspective" into the geometry that would skew any scaled measurement from a "2D" photo.
The drag coefficient often uses frontal area as a scale factor:
Cd = total drag force/ reference area/ q
So, if you know the total drag force in pounds at a given velocity (q), you could then calculate the reference area. Cd ~ .29
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