My "Planes Trains Automobiles" Contest entry was an experiment in displacement mapping...
The body of the car is just a simple spline form...
All the surface details... the doors, the handles, the windows, the rubber and chrome trim... that is all made with displacement maps.
There isn't an easy way to manually paint such maps in a paint program but it was easy to model the shapes on a flat plane. I put a white-to-black gradient on them and shot the arrangement with an orthogonal camera.
That render to OpenEXR format was applied to the body to be the displacement map. I made maps that way for the front and back details also...
I recolored the patches of that details model with white, gray and black to render versions of the map for Transparency and Reflectivity...
A turn-around of an earlier WIP version of the car...
155Lefts_3500.mp4