I think we can all agree that Rockstar continually botches car ownership. Somehow, 360/PS3 GTA games took a step back from even GTA San Andreas. What's especially frustrating is how other games like Saints Row got it right from the start of last generation. At least in San Andreas, you could find a car and stash it in one of your garages for later, and if you had all of the properties around the map that was pretty easy to store up a bunch of unique cars.
Then GTAIV came out and completely obliterated car ownership. Just... totally trashed the idea by eliminating garages and randomly spawning a shitty car in front of your residence... which made no sense to me. GTAIV did car management worse than any open world driving game ever made, worse than San Andreas, Vice City, and GTAIII.
GTAV improved things from GTAIV but still not up to snuff with even the basic system that Saints Row #1 had in 2007. Even if I knew Rockstar's system I wouldn't trust it. Can someone answer, if I buy a fancy car on one of those websites, it gets delivered to my garage, right? If I take that car out for a mission and leave it somewhere, is the car then gone? When GTAV came out this was super glitched up and didn't work and there was a lot of misinformation, and then GTA Online introduced a mostly working model of car ownership (still not very good, still not as good as Saints Row #1 in 2007, but better).. And so I'm confused how GTAV SP handles it.
Haven't read every page since but either to answer or just verify others if anyone else answered.
Cars parked in Garages, Safehouse parking spots, and Impounds will never disappear. They stay there forever*.
If you drive any car to a mission, and you cannot use it on a mission, it will always go to impound and stay there.
Some cars you can use on a mission. It depends on the mission. For example, during By The Book, it will always spawn the Bodhi for Trevor because he uses that on the mission but when you come out of the basement, technically your car will still be there -- you're just required to use it. The meet up for Blitz Play preparation also will spawn your Bodhi after the mission, even if you drove their in a different car. Three's Company, for example, forces you to even use certain clothes (normally missions will remember your customized clothes but that one forces T and F to always wear the same outfit, same for Ending C and some other missions).
Other missions let you use your car. Trevor Philips Industries let's you use your own car the whole mission. Heist 2 remembers your car to the very end (M and F drive off it whatever T was driving). Sometimes you need to use a trick... for example, Crystal Maze will despawn your car if you leave it at the 'vantage point' and then you're stuck with the ATV at the end. Even if you get back in your car and drive it closer to the house, it despawns. But I tested it like 10 times, and if you drive your car to the spot where the the ATV spawns, it actually remembers your car.
I show this in this video here (should be no ads, I quickly turned them off for GAF). You can see that instead of the ATV, I tricked the game to keeping my Dubsta car there.
If you abandon a non-customized car, it disappears. But if you customize a car and abandon it, it will always go to the Impound.
If you give a car to someone, switch characters, it will remember that car unless, 1. you restart the game, 2. switch back to them and you get a Switch Scene which has a default car (e.g. Trevor chasing someone/getting chased will always have the Bodhi by default), 3. you start a mission.
Anyhow, there are ways to keep your cars safe, and once you know what works, what doesn't... it's pretty reliable to keep cars safe. I always keep the same cars all game... I'll get very unique cars that only spawn once and keep them in garages, use them on missions, etc for months and I've done this on maybe 8-9 playthroughs now.
But there's no general rule for using them on missions and you have to know from experience which missions or events can cause problems. Sadly. It's not optimum. I do like the cars get destroyed instead of infinitely respawning on a list (a la Sleeping Dogs) but I do wish you could change your personal car or manage 'Personal Cars' to decide which will currently appear for cut-scenes, etc.
*there were lots of Garage bugs for a month at PS3 release but I never saw one again for 11 months... until the PS4 release. There is a rare new Safehouse garage bug that seems to be related to spawning your Personal Vehicle and Saving... sometimes when your personal despawns and respawns , it'll make your garage car disappear. Also, sometimes if you Save near a Safehouse where your car in the parking spot would 'draw', and your reload the save, it'll disappear... and it'll disappear every time you reload the game file..
But! if you reload the entire game by closing the app and relaunching it, it will remember your car! so... if you do lose a car in your garage, try that. don't use reload the save file. reload the game from scratch. that has worked for me..
and the new safehouse parking spot garage seems rare, I'm about 70 hours into the PS4 version so I've probably 'drove back' to my Safehouse over 100 times, so I've successfully had my car stored there safely at least 100 times. But I think... 5 times now, I've had it disappear. 3 times I could recover by restarting the game app. 2 times I could not.