Depends on the car. Our '81 VW camper is loosy-goosy enough that without the parking brake it will creep its way down our angled-but-not-anywhere-near-45-degrees driveway. On any kind of an incline, I don't trust the compression leakage on even a new car. But that's for manuals that rely on engine compression to to hold them in place. Automatics (at least the ones I've torn apart) have a pin to lock the transmission, and not rely on rolling up against engine compression.