The Nissan GT-R, Toyota Prius and Mercedes-Benz S400 Hybrid are a few of the more aerodynamic cars available today based on their coefficient of drag. We hesitate to say they are the "most aerodynamic" because we don't have drag coefficients for every new car out there.
Coefficient of drag (Cd) is the most common term to describe vehicle aerodynamics and is a measurement of how well air flows around a vehicle. A lower number, such as 0.28, indicates that the engine doesn't have to work as hard to maintain a certain speed as a vehicle with a Cd of 0.38, potentially reducing fuel consumption.
Drag coefficient is one measurement of a vehicle's ability to slice through the air, and it does not take into account the size of the vehicle's frontal area, which also affects aerodynamic drag. It also is a number that can be manipulated by an exterior nip here and a tuck there. For that reason, some automakers avoid releasing a coefficient of drag for some or all models because they fear they will get blown away by rivals bent on claiming a more slippery shape.
The Audi A6 sedan and Nissan GT-R sports car are among the more aerodynamic cars with a drag coefficient of 0.26, but the Toyota Prius hybrid is a hair better at 0.25. Nissan's goal with the GT-R's styling was to maximize speed, and Toyota's was to maximize the fuel economy of the Prius, so they reached similar goals for different reasons.
Among other cars with low Cds are the Ford Fusion, Lexus LS 600h and ES 350, Mercedes S400 Hybrid and the Toyota Camry Hybrid, all of which are rated at 0.27 by their manufacturers. In comparison, the Nissan Leaf electric car has a 0.28 and the Ford Focus SFE 0.295.
You can find other aero cars by diving deep into the specification charts for individual models in the Cars.com Research section. Most cars these days come much closer to 0.30 than 0.40 (even the Chevrolet Tahoe SUV is 0.36), so you aren't going to find many four-wheel bricks among 2013 cars.
Though aerodynamics play a role in fuel economy, vehicle weight, the size and efficiency of the engine, tires and other factors are important, as well, so a low Cd doesn't guarantee high gas mileage, as the GT-R illustrates with its EPA ratings of 16/23 mpg city/highway. A more aerodynamic shape may do more to increase a vehicle's visual appeal than its fuel economy.
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