Here is an article that lists a variety of components to drag.
Car Aerodynamics 101
Honda have made detail changes to the underbody of the 2012 Civic and changed the side view mirrors all with the purpose of reducing aero drag. It would have been interesting if they had been able to reduce the ride height at cruising speeds.
Reducing the ride height of a car is a comparatively straight forward mod, but the cost of a spring set is not inconsequential.