How do you say "how do you say car"?
You would say to your friend "como se dice car en español". Your mexican friend would probably answer "se dice carro". I believe "Coche" is used in Spain.
In Mexico the most common form is Coche and Automovil. Carro is used more a railroad car and things of that nature. Carrito is for horse-drawn carriages, shopping carts, etc.