We do fine, thank you. .....................
Sorry, silly joke.
Two ways; 1: either use a simple shape or painted area to mask the area for the smoke from above + a distribution shader to mask the height (if necessary), and add a very low cloud in that area.
Method 2: use a camera pointing backwards from the car, feed it into an image map of a small white dot in a black square, masked by a distance shader (from same camera) to set the extent of the smoke behind the car. And add cloud in that area.
Oh, and with a transform merge shader you can double that for the 2 wheels.
