As Joesph answered, simple past tense is appropriate. Why? It sounds better to my ear as a native speaker, but that's not a good explanation. We would assume that the knowledge of my five cars, if you knew, would not be forgotten. Using the present perfect tense suggests you may have known at one time, but forgot. That's unlikely. The act of knowing was one time.

have you known that i have 5 cars? since this form (which is present perfect) is used with past actions with no indication of time wheather the other form "did you know that i have 5 cars?" is simple past and it is generally used with past action with the indicator of time! Good luck

Did you know I had five cars

Did u know i have 5 cars