It's a different world here in Ecomodding Central. It's a well-known story in the Aerodynamics subforum.
A friend of
aerohead's built it in the last century. He drove it to Mexico and back once a year. You can dig out the details there. I'd be skeptical of a 20% improvement since the bottom is left undone. The point though, was about the box cavity. Here's a less extreme example:
If you care, the more extreme example would be Morelli's Urban Car.