After finalizing the effects we planned to do for our commercial last class, I have started R&D for a car exhaust smoke effect in Houdini. I have created a very basic sim coming out of the exhaust pipe and the next few weeks should be fine tuning the effect. Currently, some things I am looking into is getting the sim to dissipate at a closer distance, making it thinner in general, and adding a slight wave to it so it looks like air is pushing the exhaust around.
The smoke in my reference is very thin and barely noticeable. It is much more noticeable in motion, so I suggest viewing the video in this post.