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At 30 mph, no full-size car is operating in laminar flow; the Reynolds number at any point on the body except the very front is too high for that.
When flow trips from laminar to turbulent, drag suddenly drops and then continues to rise on a velocity-dependent curve.
In this turbulent-flow regime, Cd is proportional to the inverse square of velocity--but this variation is so small that it can be considered constant for road-vehicle speeds.
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