I had a Bullet and Comet in Michael's garage. Had them there ages, they were always there.
40 to 50 hours of gameplay they were there, pretty much since the beginning of the game. I go to the garage not even 5 minutes ago, and they are gone.
I kind of wish you could change a character's main car. The only thing you can do with cars you trick out and spend thousands and thousands on is cram them in a storage garage, or put them in your home garage just to risk disappearing. REALLY makes me feel like spending million(s) on a car online just to have that happen.
Really though, Michael's main car is just ridiculous. Looks like something a short, fat, bald, limp-dicked Jew would drive. Then again, it was his dumbass curly headed son that did it. Wish I had a actual goddamn say in it.