[Diesel_Dave]

Yes, technically, Cd does depend on Reynold's number, surface roughness, etc, etc., but those effects are pretty small. It's a fairly common assumtion that Cd is constant for a given shape, however. That's why there can be a Cd quoted for a given car. Technically a car will have different Cd's at different Reynold's numbers (and other factors), but often just assumed to be constant because those effects are fairly small.