Hi Richard
I too, didn't understand how people made these 3D renders in real images, so I did some self study during the past couple of weeks, below here I'll explain how it's done.
You need HDRI light, and matching backplates (2D photos from the same scene)
http://www.hdri-locations.com has a lot of great stuff.
Choose an image like this, and look at it for a while.
Next, you take your 3D car, and place it in the distance/angle where it will fit on the road.
In your 3D program, or Maxwell Studio, adjust the HDRI scene to macth the 2D backplate, in order to get the right reflections and shadows. Below the car, place a surface with an asphalt like material, and turn the shadow channel on. Enable the shadow and alpha channel for the render.
Alpha channel to crop out the car. Only problem is, that the road is part of the scene.
Compose the alpha with the shadow, to use as a tool to crop out the car. You can skip this and the last task, if you choose to crop out the car manually.
Cropped out car in 2D backplate. Quite obvious how the whole secret is the shadow! It can be hard to believe, but this is the final image, only thing missing is the shadow pass!
Shadow channel.
Shadow pass composited using multiply as the blending mode in Photoshop. You can link the car and shadow layers, it makes it a lot easier to move around and scale the car. You have to adjust the layers colors, contrast, midtone contrast etc. to make it look as real as possible. Now you're done!
