"We talk about my car."
Translation:Wir sprechen von meinem Auto.
Von is always followed by a dative, über is a "two-way" preposition - if it's used to state a (static) position, it's dative ("Über den Wolken", song title...), otherwise it's accusative, like here. One more example:
- Ich hänge die Lampe über den Tisch ("over the table", acc., motion/destination)
- Die Lampe hängt über dem Tisch ("over the table", dat., position)
However, in the sense of "about", über is always followed by an accusative, like here.