It's not just the length of the side skirt. If you look really close to a Black Widow side skirt, you'll notice that there is a part of the skirt that matches up to the door of the car. So if you have a coupe, the "groove" in the side skirt is matched to the length of the door. And since the door on a sedan is shorter than a coupe, that "groove" would not match, even if it was customized to fit.