In spanish, we make differences if it is about one thing (car = auto = carro = coche) or more than one thing (cars = autos = carros = coches). If we are takling about one single car: my car - mi auto your car - tu auto/ su auto (de usted, it is more formal) his car - su auto (de él) her car - su auto (de ella) its car - su auto (de algo) our car - nuestro auto (de nosotros / de nosotras, if we are all women) your car - su auto (de ustedes/ de vosotros) their car - su auto (de ellos/ de ellas) so, "su auto" can be: your, his, her, its or their car. If we are takling about two or more cars: my cars - mis autos your cars - tus autos/ sus autos (de usted, it is more formal) his cars - sus autos (de él) her cars - sus autos (de ella) its cars - sus autos (de algo) our cars - nuestros autos (de nosotros / de nosotras, if we are all women) your cars - sus autos (de ustedes/ de vosotros) their cars - sus autos (de ellos/ de ellas) so, "sus autos" can be: your, his, her, its or their cars. I hope it can help you.

Su coche es viejo. His car is old. Tu coche es viejo. your car is old. Sus coches son viejos. His cars are old (plural of the first sentence). He have two or more cars.

"Your car is old and his cars are old"