Acid is very soluble in water, so it should be very easy to wash off. If you want to be doubly sure, then wash the whole bottom of your car with a solution of baking soda and water. Put a whole box of baking soda into a barrel of water, mix well and then wash the car bottom thoroughly with this. The baking soda is a base and will neutralize any remaining acid. You may also be concerned about the acid eating away the paint, or some other coating and exposing some part of the car to ordinary wear and tear from the elements like air, water, sand and salt. It might be a good idea after cleaning it really well, to dry it well, re-grease and re-paint to protect the chassis.