After everything was finished, I started tweaking the render settings for better quality shadows and glass reflections. The overall tweak I made rotated the sky dome in order to get a good reflection from the HDR image, and I also added a floor to capture the shadows on my shadow and occlusion pass.
The final image, as well as all the render passes, was rendered using Mental Ray. The settings were simple: Final Gathering was on, with Accuracy: 200; everything else default. The Raytrace/Scanline Quality was set to adaptive sampling, min 0 and max 2. The rest was all default, as well.
To render an occlusion map, first of all select your camera, go to environments and change the background colour to a full white. Select all the pieces of your scene - in my case the entire car plus the floor - and then add the node "surfaceShader" to them. Create one Ambient Occlusion shader; to do this, on the Hypershader go to: Create, Mental Ray Nodes, Textures, "mib_amb_occlusion". Connect the "Out Color" of the node you just created to the "Color" input of the "surfaceShader". That's all! Now you can render the image and save the result as Tiff or Tga format.
Usually I render a few different passes of an image in order to be able to tweak the overall look without having to re-render the entire scene. First I render one pass of each kind using just one light with the HDR image, like: Shadow, Beauty, Occlusion and Diffuse. Then I try different settings with several lights in order to get a high range of images, so I can merge them all in post production using Adobe Photoshop, or Adobe After Effects for animations.
I spent one or two weeks working on the sketch and modelling all the pieces, plus several weeks working around major problems with the lights and re-designing a few parts of the car just by eye in order to make it look "right". Once it was almost finished, I went forward and modelled the insides as well, like the seats, driving wheel and mirror.
The final result became my favourite and best work, so far. The funny part of this project was that I almost gave up! There was a moment when nothing seemed to be working; the lights didn't work as I expected; the HDR and reflections weren't looking as I wanted; the occlusion map was not helping at all; the car shape wasn't looking as I wished it would... but thankfully, I didn't give up - I kept trying, re-designing and experimenting with different settings, even when my own mind was telling me it was a lost project, beyond repair. At the end, I found my balance between the shape and lights and was able to conclude the image.