How do you clean exhaust residue on a car?

If its on a painted surface, be careful.

You can try first using the "sap and tar" remover, with a little light rubbing.

Chances are, though, that the exhaust has 'cooked' itself into the clear coat, and won't come off with the remover. If that's the case, wash the ground effect real well, and then use polishing compound. Using a clean, damp cloth, apply the polishing compound and use light pressure to get the exhaust crud off the clear coat.

When you're done, put a couple of coats of wax on your ground effect - your standard Turtle Wax stuff. Coat/polish/coat/polish/coat/polish. That way its easier to clean off the next time.

When all else fails try using a little diesel petroleum. This stuff will take the Grey out of concrete if you let it. Use very sparingly (stingy).

Just a guess you might not want to do this but i know with getting things like resin off things you use rubbing alcohol.