Aerodynamics is a funny science. What the eye tells you is slippery may not be. Check out this article on the Porshe 993, a 911 variant:
http://autospeed.com/cms/A_108675/article.html
Going back to the Turbo 911s of the '70s you see a big-assed spoiler on the back. It's not there for looks and it's about the size of a trunk deck. Hmmm...
Keep reading further down the article about the Beetle (like the one I have) and you'll see the shape can cause a surprising amount of lift and drag. I'm looking into a spoiler to put back there now.
What body style is it? Sedan, coupe, hatchback?