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It's worth noting that there is no standard that manufacturers must use to measure either Cd or frontal area. One rule of thumb, w x h x .81 is incorrect for most cars, (because it does not account for differences in roundness or degree of trapezoidal shape of the greenhouse). Many manufacturers use the actual frontal area of the car and its tires, and do not include any part of the air under the car. Mirrors are often, but not always, included.
In a wind tunnel, Cd cannot be measured directly. It is calculated from drag force (which is measured directly) and frontal area. If two manufacturers happened to make exactly the same car, but one uses mirrors in its frontal area, and the other does not, then the Cd values will be different.
The Prius was measured independently at .30 by a contractor to Chevy (IIRC) when the Volt was being developed. The Toyota number is .25, which I don't doubt. Pretty huge differences for the same car.
Wind tunnels can have either stationary floors or be moving floors, and calculations can be adjusted accordingly (or not). Even when adjusted, the effects are not easily and accurately transferred from one car to another, because the under-car flow effects different cars in different ways.
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