Actually that's completely not how cars are designed. Design comes, then come the engineers, then come the clerks, multiply that by a lot of iterations and you got the car which looks far different from the first concept.

A car is a great example how a fantastic design in terms of aesthetics can fail against engineers' (or economics) limitations. A quite recent example: BMW X6's tail lights were sent back to redesign after engineers found out that one 'beauty' line caused Cx (aerodynamics factor) increase. Imagine it happens all the time during the design process which takes years, not days.