Air resistance: why speed changes consumption

Useful to know

The moving car interacts with the air around it and has to overcome aerodynamic resistance. It is one of the forces that affect the necessary energy, but it is not the only one. There is more rolling resistance, changes in height and an acceleration cost. Therefore, a lower drag coefficient is not an automatic proof of a lower total consumption in any situation. It describes a certain part of the physical picture that must be considered with the other conditions. [1]

The general resistance equation contains the density of the air, the square of the speed relative to it, a selected reference area, and the coefficient of resistance. NASA presents this dependence as D = Cd × ρ × V² × A / 2. With other values unchanged, doubling the speed gives a four-fold resistance force. This is a consequence of the mathematical model, not a promise that the total cost of any car will necessarily grow exactly fourfold. [1]

The speed in this model is relative to air. Air mass movement matters, so a uniform speedometer reading does not guarantee completely uniform aerodynamic conditions. Even without making accurate calculations, this keeps us from jumping to conclusions after two different trips. For a meaningful comparison, it is necessary to take into account the environment. A single observation in different wind, load or route does not isolate the effect of the body shape.

The Cd factor should also be read carefully. NASA clarifies that it is related to the selected reference area. In the same way of determination, a lower Cd is a useful characteristic, but the strength also depends on the area A. Two cars of different sizes cannot be compared completely by their coefficients alone. The larger body may have a well-organized flow and still encounter significant resistance because of its size. [2]

A practical example is a cargo box on the roof. FuelEconomy.gov points out that the additional load there can increase aerodynamic drag and lower fuel economy, especially at high speed. Here not only weight matters, but also the change in the external shape. Therefore, the cost with a trunk should not be attributed entirely to the kilograms of luggage. Removing unused external equipment is a different measure from unloading items that remain inside the bodywork. [3]

From this physics follows a better way of reading automotive characteristics. First it is specified what the number measures, then under what conditions it was obtained and finally whether the cars compared are comparable. Aerodynamics is related to both gasoline and electric cars, because air acts on the moving body regardless of the source of energy. For the daily driver, the most useful conclusion is not to separate speed and external equipment from the issue of cost when comparing results from different trips.