I don't know how much you've lurked here, so I will repost a few things, again.
This is a wheel spat that consists of a metal strap and that
other wonder material, conveyor belting. Two U-shapes connected on their long edges so the belting can flex if necessary. The belting would want to curve more than bend, so the sketch isn't too accurate. As many as one or two on all four wheels.
For the rear, a diffusser. They require a smooth underbody airflow. Here are two extreme example to show the features:
Outside fences that extend to the axle line, to separate the wheelwell airflow from the underbody flow.
The inside fences exist because the outside channels are more turbulent. Three or four channels are more typical. Note that the outer fences flare outward. The slope of the diverter floor and the angle of the outer fences make a divergent duct.
Here's a picture of Bombshell Betty as a bonus:
