Some vehicles have a body design where the front bumper doesn't redirect the air around the wheels as well as it could. Any air that flows into the tread (which is moving around the wheel as fast as the vehicle is moving) can cause a lot of drag, and this can reduce performance and fuel mileage. To avoid this, you can add a wheel well fin to your vehicle, which can improve front downforce and reduce drag.
Does Your Vehicle Need One?
Not all vehicles have bumpers which leave tread showing from the front view of the vehicle, so check for the flaw before you consider making corrections. Many sports cars don't have this design issue.
Designing the Wheel Well Fin
The fin should be connected to the bumper in a way that will keep it from bending in the wind, and not stick out further than it needs to. It should be designed so oncoming air will hit the fin instead of the wheel.