A un o no a un
 

As an English speaker I would definitely say "If I had a car...." since "If I had car...." would sound terrible, but of course would be understandable.

¿Por qué en Español?

Si tuviera coche podría visitar a mis amigos.
Si tuviera un coche yo podría visitar a mis amigos.

Como siempre, gracias.

The construction "tener coche" is possible because it corresponds to a social stereotype or something that may be somehow generalized, characteristics or habits shared by many subjects:
- ¿Tienes perro?
- Nunca uso abrigo.
- Ya nadie lleva sombrero.
- El edificio tiene calefacción.

However, if you were talking about a characteristic that cannot be generalized, you must use an article:
- Juan tiene una serpiente. (While many people have a dog, it's not common to have a snake.)
- El edificio tiene una grieta en la pared. (Every building can have a heating system, but not every building has a crack on a wall.)

Gracias otra vez Angelica.

I'm glad to help. :)