The only difference is they're used in different places. Coche in Spain, Carro in Latin America.
Also carro in Spain is for example what farmers used to tie to horses for them to pull it, and in Latin Spanish means car. In Spain car is named coche.
Think the carro refers to a wagon and are also used for shopping cart(carrito). Coche is a more modern version.. Not native spanish but i live in spain
Yeah, you would put the yo mostly to clear up who you're talking about, or to put special emphasis on yourself.
When are the bosses at DL going to clear up this confusion? The Castillian Spanish (proper Spanish) word is COCHE. The South American word is CARRO. As we are supposed to be learning Castillian Spanish, since the DL "upgrade" a few weeks ago there have been numerous SA words introduced - all they do is confuse ! 20th Sept 2018.
So that is why they have changed! Actually we used to learn coche for car here at Duo so I was surprised when all of a sudden it was supposed to be carro. For me living in Spain a carro is something else, that is what tourists are pulled around in behind the poor horses or donkeys. The origin of the car, I suppose!
Viejo if the noun it describes is masculine, and vieja if the noun is feminine. The noun that's being described here is carro, which is masculine, so you use viejo.