Technically, if an object is moving aerodynamics will affect performance. Because flow restriction of a fluid is proportional to speed squared, it generally just kind of works out that around 30 miles an hour there is a significant increase in how much affect further increase in speed will have. Most cars are designed to optimally operate around 30-40 miles per hour, as that tends to be the average speed for driving in cities, where gas mileage is worst. In rare types, such as solar race cars, the car is designed to run optimally at highway speeds with electric motors. Formula cars are optimized to run at speeds in excess of 100 miles per hour to achieve peak engine efficiency. The aerodynamics can be adjusted depending on the designed function of the vehicle, however other design factors often come into play, such as cargo capacity, driver comfort, passenger number, and so on. For instance, getting a vehicle to comfortably seat 6 people that has the same aerodynamics as a solar race car would be nearly impossible because of the minimum aerodynamic cross section of the vehicle. This cross sectional area is one of the major factors that will affect aerodynamics.
Kansas State University Solar Car Mechanical Team leader.