Do the following if you get a vehicle you really like and you do not want to risk losing it in an accident, or you want one for every safe house. After unlocking access to Algonquin, take your car and drive over to your South Bohan safehouse. Once there, drive east toward the Northwood Heights bridge that leads into Algonquin. From there, turn around and drive back over the Northwood Heights bridge. After you get across, turn right onto San Quentin Ave., in Fortside. As soon as you make the turn, you will notice a couple cars identical to the one that you are driving, just in varying colors. To save them, just alternate between cars every block until you get to your South Bohan safehouse to store one and drive the other one where ever desired.
If you happen to come across a rare car (Infernus, Coquette, Super GT) and want more of them to keep, drive up to the safehouse in Bohan. There will be at least ten of them lined up around the safehouse, just for you.