Here is a drag table from FIAT from around 1986.It illustrates the influence of body greenhouse tumblehome for three different body types and their respective reaction.
Tumblehome is a term borrowed from naval architecture to describe the shape of the sides of ship hulls.'Old Ironsides,' the U.S.S. Constitution is an example of a ship with tumble home,which influenced it's 'armor' capabilities.
You'll notice from the table that fastback cars benefit the most,and squareback cars (like the Nissan Cube and KIA Soul ) actually suffer a drag penalty from it.
If you are contemplating some boat-tailing,then tumblehome should be included in the build.
