Right now, there are fuel mechanics, repair mechanics, and in my opinion they kinda of make owning a personal car (that should be a convinience, as you dont have to worry about the law and can mod it as you please without the worry it will despawn) more of a hassle, and heres why:
1-Fuel mechanic is useless when you arent driving something you own (Why refuel this car I robbed? Lets just jack other.) and is boring, in my opinion, to watch out for. I dont want to drive into a gas station every time I do a big travel around LS. In my opinion just remove it, or add a (one time buy) upgrade for garages that allow them to refuel cars for free.
2-Having to go to a garage and pay to repair a car, every single time you crash it to look good is a hassle in a game called GTA where you spent 45% of the time you drive crashing your car (I dont mean that cars should be invulnerable, but maybe allow players to hire permanently (one time buy) a mechanic for garages that repairs your car for free every time you send to that garage (on a property or individual garages).
This is my opinion, maybe I'm wrong about it, let me know please.