Q: What do rear fins do on a Model Y?

As air travels down the body it can peel away too early and spill into the wake, which creates drag and a less settled feel. Mounted at the outer edges of the rear bumper, these fins act as subtle fences that help the airflow stay attached a little longer before it enters the wake. Keeping the side flow organized supports pressure recovery at the rear, so you get calmer air, better highway stability, and a more planted rear end.