Is it safe to clean concrete off a car with muriatic acid?

YES!!!!!!! It has to be diluted with water or it will eat away at the surface. I know from personal experience.

This is very strong and corrosive. and should never be used full strength unless you are trained in using.

Muriatic acid is an impure form of hydrochloric acid. It is extremely corrosive and it is advised that you use a face shield, and use in a ventilated area only. The fumes are very corrosive and can cause irreparable lung damage. Improper use can release chlorine gas, a gas used in WW I by the Germans. If the purpose is to clean brick, or etch concrete it can be diluted, and should be. The recommended concentration to clean brick is at least 10 parts water to 1 part acid. If the acid releases a very strong fume on opening use 15 to 20 parts water.

It also reacts with various metals to form an explosive mixture with air.