Thedrag coefficient is a common measure inautomotive design as it pertains toaerodynamics. Drag is a force that acts parallel to and in the same direction as the airflow. The drag coefficient of an automobile measures the way the automobile passes through the surrounding air. When automobile companies design a new vehicle they take into consideration theautomobile drag coefficient in addition to the other performance characteristics. Aerodynamic drag increases with the square of speed; therefore it becomes critically important at higher speeds. Reducing the drag coefficient in an automobile improves the performance of the vehicle as it pertains to speed andfuel efficiency.^[1] There are many different ways to reduce the drag of a vehicle. A common way to measure the drag of the vehicle is through the drag area.

The reduction of drag in road vehicles has led to increases in the top speed of the vehicle and the vehicle's fuel efficiency, as well as many other performance characteristics, such as handling and acceleration.^[2] The two main factors that impact drag are the frontal area of the vehicle and the drag coefficient. The drag coefficient is a unit-less value that denotes how much an object resists movement through a fluid such as water or air. A potential complication of altering a vehicle's aerodynamics is that it may cause the vehicle to get too much lift.Lift is an aerodynamic force that acts perpendicular to the airflow around the body of the vehicle. Too much lift can cause the vehicle to lose road traction which can be very unsafe.^[3] Lowering the drag coefficient comes from streamlining the exterior body of the vehicle. Streamlining the body requires assumptions about the surrounding airspeed and characteristic use of the vehicle.

Cars that try to reduce drag employ devices such as spoilers, wings, diffusers, and fins to reduce drag and increase speed in one direction.^[4]

While designers pay attention to the overall shape of the automobile, they also bear in mind that reducing the frontal area of the shape helps reduce the drag. The product of drag coefficient and area – drag area – is represented asCdA (orC_xA), a multiplication ofCd value by area.

The termdrag area derives fromaerodynamics, where it is the product of some reference area (such as cross-sectional area, total surface area, or similar) and the drag coefficient. In 2003,Car and Driver magazine adopted this metric as a more intuitive way to compare the aerodynamic efficiency of various automobiles.

The force F required to overcome drag is calculated with thedrag equation:${\displaystyle F=\frac{1}{2}\times \text{air density}\times \text{drag coefficient}\times \text{reference area}\times {\text{speed}}^2}$Therefore:${\displaystyle F=\frac{1}{2}\times \text{air density}\times \mathbf{\text{drag area}}\times {\text{speed}}^2}$Where the drag coefficient and reference area have been collapsed into the drag area term. This allows direct estimation of the drag force at a given speed for any vehicle for which only the drag area is known and therefore easier comparison.As drag areaCdA is the fundamental value that determines power required for a given cruise speed it is a critical parameter for fuel consumption at a steady speed. This relation also allows an estimation of the new top speed of a car with a tuned engine:

${\displaystyle \text{estimated top speed}=\text{original top speed}\times \sqrt[3]{\frac{\text{new power}}{\text{original power}}}}$

Or the power required for a target top speed:

${\displaystyle \text{power required}=\text{original power}\times {\left(\frac{\text{target speed}}{\text{original speed}}\right)}^3}$

Average full-size passenger cars have a drag area of roughly 8 sq ft (0.74 m²). Reported drag areas range from the 1999Honda Insight at 5.1 sq ft (0.47 m²) to the 2003Hummer H2 at 26.5 sq ft (2.46 m²). The drag area of a bicycle (and rider) is also in the range of 6.5–7.5 sq ft (0.60–0.70 m²).^[5]

The average modern automobile achieves a drag coefficient of between 0.25 and 0.3.Sport utility vehicles (SUVs), with their typically boxy shapes, typically achieve aCd=0.35–0.45. The drag coefficient of a vehicle is affected by the shape of body of the vehicle. Various other characteristics affect the coefficient of drag as well, and are taken into account in these examples. Many sports cars have a surprisingly high drag coefficient, asdownforce implies drag, while others are designed to be highly aerodynamic in pursuit of a speed and efficiency, and as a result have much lower drag coefficients.

Note that theCd of a given vehicle will vary depending on which wind tunnel it is measured in. Variations of up to 5% have been documented^[6] and variations in test technique and analysis can also make a difference. So if the same vehicle with a drag coefficient of Cd=0.30 was measured in a different tunnel it could be anywhere fromCd=0.285 toCd=0.315.

• Automotive aerodynamics
• Drag (physics)
• Drag equation
• Paul Jaray

• Further 500 drag coefficients
• Improving Aerodynamics to Boost Fuel Economy
• Tel Aviv University reduces drag on trucks by 10%
• Simple roll-down test for measuring Cd and Crr for cars and bikes

• Automotive engineering
• Drag (physics)

• Articles with short description
• Short description is different from Wikidata
• Use dmy dates from January 2022