[Snax]

Diffusers are also largely considered ineffective if airflow under the car is also not smoothly managed. I.e., turbulence from under the car remains turbulence behind it, but if smooth flow can be maintained, not only can these create downforce, but reduce wake size. In most production cars as Vman455 already stated however, they are cosmetic, and even most people adding them onto their street cars are really just playing dressup without the underbody work to go with it.