What kind of aero you call effective depends greatly on the speed range. Since drag starts to really rear its ugly head around 50MPH (for sake of arguement) we are talking about the speed range between 50 to 80 MPH. In this speed range we are talking about low speed aero. We can forget about anything high speed. It's all about smooth transitions at these speeds. the optimum shape will look similar to a fish or bird body. Smooth continuous lines starting with a rounded entry and ending in a point (or truncated one). Turning air or creating turbulence takes energy. The more we can avoid sharp turns, eliminate rough spots, speed ups, or slow downs along the surface the more we reduce drag. There are subtle tricks given the space limitations but just about any car can be improved upon. And improvement equals higher MPG.
