Did you know that geese flying in formation inspired the 1934 Chrysler Airflow, perhaps the first streamlined car ever built? Aerodynamics didn't start playing a major role in automotive design until the '70s and the first gas crisis. The measuring stick for automotive aerodynamic is the coefficient of drag, which is engineering terms is expressed as "Cw." American automotive enthusiasts and manufacturers usually refer to the term as "Cd" or "Cx."
Today's cars have a Cd around 0.29, and designers are trying for 0.25. Radical concept vehicles have gotten as low as 0.19. Again, animals are being studied to help with design work. Mercedes used a boxfish when sketching the Bionic concept vehicle.
But cars can't be just aerodynamically smooth, they also have to be attractive. Hence the challenges of today's automotive designer.