See also: Automotive aerodynamics.
The drag coefficient is a common measure in automotive design as it pertains to aerodynamics. 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 the automobile 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 and fuel 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 as (or CxA), a multiplication of value by area.
The term drag area derives from aerodynamics, 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 the drag equation:
F=\tfrac{1}{2} x airdensity x dragcoefficient x referencearea x speed2
F=\tfrac{1}{2} x airdensity x dragarea x speed2
estimatedtopspeed=originaltopspeed x \sqrt[3]{
| newpower | |
| originalpower |
powerrequired=originalpower x \left(
| targetspeed | |
| originalspeed |
\right)3
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 a . 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, as downforce 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 the 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 was measured in a different tunnel it could be anywhere from to .
| 1938 | Volkswagen Beetle | 0.48[7] [8] | |
| 2018 | 0.454[9] | ||
| 2012 | 0.31 [10] | ||
| 2019 | Toyota Corolla (E210, UK) | 0.31 [11] | |
| 2001 | Toyota Prius | 0.29[12] | |
| 2005 | Chevrolet Corvette C6 | 0.286[13] | |
| 2012 | Tesla Model S | 0.24 [14] | |
| 2017 | Tesla Model 3 | 0.23[15] | |
| 2019 | Porsche Taycan Turbo | 0.22[16] | |
| 2021 | Mercedes-Benz EQS | 0.20[17] | |
| 2022 | Lucid Air | 0.197[18] | |
| 2024 | Xiaomi SU7 | 0.195[19] | |
| 1996 | General Motors EV1 | 0.19[20] |
| 1952 | 0.26 | ||
| 1933 | 0.25 | ||
| 1954 | Alfa Romeo B.A.T. 7 Concept | 0.19 [21] | |
| 2021 | Aptera SEV (2019 relaunch) | 0.13[22] | |
| 2000 | General Motors Precept Concept | 0.16 [23] | |
| 2022 | 0.170 [24] | ||
| 2013 | Volkswagen XL1 | 0.19[25] | |
| 2018 | Ecorunner 8 (Shell Eco-marathon) Prototype | 0.045 | |
| 2022 | Sunswift 7 | 0.095[26] [27] |
| sqft | m2 | Automobile model |
|---|---|---|
| 3abbr=onNaNabbr=on | 2011 Volkswagen XL1 | |
| 3.95abbr=onNaNabbr=on | 1996 GM EV1 | |
| 5.52abbr=onNaNabbr=on | ||
| 6abbr=onNaNabbr=on | 2001 Honda Insight[29] | |
| 6.05abbr=onNaNabbr=on | 2012 Tesla Model S P85 | |
| 6.2abbr=onNaNabbr=on | 2014 Toyota Prius | |
| 8.79abbr=onNaNabbr=on | 1956 Citroën DS Spécial[30] | |
| 13abbr=onNaNabbr=on | 2019 Ram 1500[31] | |
| 17abbr=onNaNabbr=on | 2013 Mercedes-Benz G-Class[32] | |
| sqft | m2 | Automobile model |
|---|---|---|
| 0.21abbr=onNaNabbr=on | Pac-car II[33] | |
| 2.04abbr=onNaNabbr=on | 2011 Aptera 2 Series[34] | |