What is drag coefficient (Cd) and why is it important for vehicle design?

Drag coefficient is a dimensionless number representing aerodynamic resistance, typically ranging from 0.25-0.40 for modern cars. Lower Cd means less air resistance, improving fuel efficiency and top speed. At highway speeds, aerodynamic drag is the dominant resistance force. Reducing Cd by 10% can improve fuel economy by 3-5%. Modern efficient sedans achieve Cd around 0.25-0.28, while SUVs are typically 0.35-0.40.