My mom is the one making the payments and paying off the insurance on my car (a 2002 Hyundai Santa Fe). She asks to drive it every now and then, which is fine, I don't mind if my mom drives it, or my aunt, or my uncle... however, I do not want my stepfather driving my car.
There is a reason for this: when we bought the car, it was used, so it wasn't brand new, but it is a very nice vehicle. Nothing broken or messed up on it. I added my own personal touch with the shoulder-guards for the seat-belts (the seat belts irritate me so I have to use the guard things). I'll admit, I do tend to use the floorboard as a junk room, and often have empty drink bottles and bits of paper tossed in the floorboards, but I don't let it get crazy or anything.
However, whenever my stepfather drives my car, he does things that he knows I don't want him doing in it and he has no respect for me asking him to refrain from doing things. I hate anyone smoking in my car, as I hate the smell of cigarette smoke, it bothers my breathing. I ask my stepfathre to not smoke in my car, but he does it anyway, cigarettes and 'legal marijuana'. He also never puts gas in my car and most times when I get it back, the gas-light is on. All I ask is that he put as much gas back in it that was in it when he borrowed the car. I also keep several dollars in change in my car for emergencies, and he spends it all of him whenever he drives it, even though I've asked him not to. He buys cigarettes or washes the money at a car wash. Also, and this is what gets me, he has no respect for MY car. He breaks things in my car whenever he drives it, like the air vents on the dash (and there's no reason to even touch them!), and then he doesn't even tell me and let's me find out when he brings my car back and is gone.
I don't know how to tell my mom, who is making all the payments on the car, that I don't mind her driving the car, but I don't want my stepfather to drive it because he doesn't respect me enough to take care of my car. Whenever I have had to borrow and drive their cars in the past, I've kept their car in the exact shape they were in when I got it, took it back with the exact same amount of gas in the it as it was when I borrowed the car, and never touch any money they had stashed in their car and I definitely have never broken anything in their car!
Edited to add:
My stepfather does the same thing to my aunt and uncle's car (my aunt and uncle are living with my mom and stepfather until they get a place of their own). He'll take their car out, and bring it back beat to hell and back. They only bought the car a few months ago, and already it is dented, banged up, scratched, things broken inside it, etc... My aunt and uncle, though, have to put up with it because they are living there until they can find a place of their own.