In general, Cd is not an absolute constant for a given body shape. It varies with the speed of airflow (or more generally with Reynolds number).

Now that I'm on my laptop instead of my phone, I'm seeing the much better comeback, from the previous section:
Cd is not a constant but varies as a function of flow speed, flow direction, object position, object size, fluid density and fluid viscosity. Speed, kinematic viscosity and a characteristic length scale of the object are incorporated into a dimensionless quantity called the Reynolds number Re. Cd is thus a function of Re. In a compressible flow, the speed of sound is relevant, and Cd is also a function of Mach number Ma.

For certain body shapes, the drag coefficient Cd only depends on the Reynolds number Re, Mach number Ma and the direction of the flow. For low Mach number Ma, the drag coefficient is independent of Mach number. Also, the variation with Reynolds number Re within a practical range of interest is usually small, while for cars at highway speed and aircraft at cruising speed, the incoming flow direction is also more-or-less the same. Therefore, the drag coefficient Cd can often be treated as a constant.