|My big square truck creates a lot of turbulence when it moves. Does my roof rack (a factory-installed one, close to the roof) actually improve aerodynamics, like fuzz on a tennis ball? (Also, what about the air dam at the back end?)
I'm sure that modern car designers consider aerodynamics when building a car or truck. They do structure the trailing edge of the car to minimize its turbulent wake. But I doubt that a roof rack helps much. It's probably too tall for the boundary layer on the car and extends into the free flowing stream beyond. As a result, it probably experiences its own pressure drag. The "fuzz" that trips the boundary layer has to be no taller than the boundary layer itself, otherwise it causes turbulence in the main airstream rather than preventing it. The same goes for the air dam.