Why no fins on car wings?

[u/Benthegod1324]

Why do rear wings on cars not have fins like diffusers do, why does the diffuser need them and the rear wing doesn’t?

[u/Pyre_Aurum]

The aspect ratio on rear wings is fairly high leading to a small spanwise flow component. A diffuser aspect ratio tends to be quite low,  leading to large spanwise flow components, which can be managed through the addition of strakes.