Its actually quite simple and it really makes a difference in the realistic look of the chop.Originally Posted by tvcasualty138x
You need to create a layer above the wheels and tires and below the body layer.
The use a fairly large soft edged brush (using black for your color) and draw a line with the center of your brush just above the wheel opening.
After you have the black drawn you simply turn down the opacity of the layer until it looks right.
To emphasize the technique I have moved the layer above the body so you can get an idea of what to do.
If you are working on an image that doesn't have the body on a separate layer you can create the same effect by making a selection of your wheel wells and create a new layer for the black and then use the same technique as above.