[soueuls]

Maybe because "My new car" does not imply that the car itself has never been used?

Just that it's YOUR new car.

[yard2010]

That's one thing I'm very curious about: is there a way in english to differentiate between "(my new) car" (a used car which is new to me) and "(my) new car" (a new car which is mine)?

[Sanchless]

Usually you'd say "brand new" to signal that it's from the factory. It is quite ambiguous otherwise.