I've testing around with saving cars, house garages and garages for the past hours. I thought it could be good to know how things works with the house garage / garage / car impound.
###### Houses
Every character has a different amount of vehicles. Michael have his classic Obey Tailgater car, but also deppending on which family members are home, he will have his family members cars parked at the house. Franklin always has his white Buffalo and the other vehicle depending on how far in the story you are.
Those cars can be modded and the mods will be saved. So I would suggest fixing the main cars first with body mods and armor would be a reasonable investment.
Since I've been testing with Michael and Franklin, both have house garages and to save a car, you just park it in there and then leave. If you park it, walk out get the garage door shut then grab your vehicle it will be removed from the garage but enters a "save state".
###### Garage
Have spots for 4 vehicles. Bicycles/bikes/cars. By standing outside the garage door with your vehicle you press the right arrow on the "arrow-pad" on your controller and you will be loaded into the garage on foot, and the vehicle is parked inside.
If you grab a vehicle from the garage by entering (triangle or Y) it inside, you will get loading screen and be loaded out to the street. The car will be removed from the garage but enter the "save state".
Note: If you enter on foot because your garage was full when you just was about to save a car, the car outside COULD be removed when you get back out. Happened to me like 50% of the time.
###### Car Impound
Every car that has been in the "save that" will get to the impound if you lose it. If you park a car in "save state" in any garage, that car will be removed from the "save state". You only have 2 impound slots.
Note: You can only have one car of the same type taken by the car impound.
For example:
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If you mod a Carbonizzare, let us say, full body mods, lime green color.
Save it in the garage then pick it up. It is now in the "save state". While roaming the highway, you suddenly die by some evil cop or something. The fully modded, lime green car will be stored in the car impound since it has been saved in your garage once.
An hour later or so, you find a brand new white Carbonizzare parked by a douche on the handicap parking over at Del Perro Pier, you steal this care to feel a little bit better by yourself.
Despite the white color, you want to park this car in your garage in case you need it later. You save it in the garage, pick it up again and start your daily show-off by cruising Los Santos.
Cruising went well, a lot of people took pictures of your ride and your ego is now complete. You park the car and take a walk downtown. The car is now lost. But since you made it enter a save state you will be able to get it back from the car impound. BUT!!! Your lime green, fully modded Carbonizzare are now gone. Forever.
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###### Extra
This works SOMETIMES: If switch from a character while the are driving a newly picked up vehicle that isn't saved. That vehicle will be stored on the street outside their house next time you switch to them.
Most common one seems to be someone stealing a wallet but there are tonnes of them.