[MetroMPG]

The Metro's rear bumper design makes for a bad traditional fender skirt (one that s imply fills the wheel arch). The lower ~50% of the bumper cover is tucked in behind the rear tire, and it curves abruptly away (transitioning to the rear surface). Even with a smooth surface to that point, there's no chance flow is attached there.

So, yeah, a fender skirt that extends further aft and tapers more gradually should work better.