to be honest I've never tried sleeping in the car but I guess I will have to as I will have a trip soon...
By the way I found out that 2 hours of sleeping is enough for me during the trip to feel refreshed. Moreover, it wouldn't be possible without the help of sleeping tracker that always gave me accurate data about the phases of sleep and my heart rate. In fact, I would never learn how these sleep trackers work without the help of this article
smartifylife.com where I also found out about sleep stages that helped me understand how much time is enough to feel refreshed :)