I am not exactly certain if there are any specific objective conventions set in regards to naming someone's car. I believe it must predominantly have to do with the car's personality when an owner begins getting to know her well. Generally, cars are named female. Supposedly all cars are female. Personally I've run into one example in which that was incorrect.
In my life I have owned four cars. My first car was a silver-grey Mercury Marquis. I called him Marky Mercury. I had been told that when he was new he was purchased by a police department and used as an unmarked car for drug busts and stakeouts. I'd believe it. He was built like a tank. He was definitely male but I couldn't deny it: Marky was a homosexual. He never exactly came out of the closet and told me. I just knew. His personality screamed flaming. I mean look at the name: "Marquis." He acted very much like Freddie Mercury. Very loud. Talented. Could take a beating and keep on going, but he was very effeminite. Just not in the way most cars are. He was faking it. A big tank of a car with a bit of daintyness about him, and as a white straight male college student majoring in Theater who suffered from homophobia you can imagine our relationship was often a bit strained.
Near the end of his life we got into an accident in which someone rear-ended us. All this did was bend Marky's frame up and to the left about an inch, making his trunk difficult to open and close from then on. And the back of the driver's seat was ruined, and wouldn't stay up straight without intensive mechanic work. The other car was completely totalled. My car had been buttfucked for all intents and purposes. He seemed to have enjoyed it and drove away from the wreck, but he never was the same. Though he insisted he was fine, for weeks and months after that he started slowly falling apart on me. It was like watching a dear close friend die of AIDS. I could no longer afford the bills and in the final weeks he was always in the shop. In the end I had to let him go.
My next car was a blue Ford Tempo who had a personality like a hispanic Catholic. She was very saucy and spicy but wouldn't always deliver if you know what I mean. I named her Alexa Rae Jessica Tiempo. No lie. I named her Alexa Rae after Billy Joel's daughter. Jessica is a name that's just always had a soft spot in my heart. I felt a catholic car would have a christened name, but it didn't occur to me until later that there's no saint Jessica. I'd only use her full name when she was ill-behaved. She served me well through the final couple years of my time in college and on up until my marriage. I didn't treat her well though. Finances were never in such a way that I could easily afford to give her regular tune-ups. She got old before her time and it was largely my fault. A lack of proper upkeep, but she never held it against me.
I turned her in as a trade at about the same time I got married, and purchased my first Saturn. She was red. Great car, but I hadn't learned my lesson from Alexa Rae. She too suffered from poor maintenence, but Saturns are troopers. They're built like efficient marines. To the bitter end she was dependable and hardworking. She was all business. Not much on personality. She stood at attention and did as she was ordered to do, with a sense of duty that only Saturns possess.
She never told me her name. I never gave her one. She was simply The Red Saturn.
One ill-fated night in a brisk February sleet-rain, we hit a patch of black ice while driving north on the Dallas Tollway; A patch of black ice that neither she nor I could see. We slid left. I tried to compensate. We slid right, hit a guard rail and before I knew it we did two 360s. Every corner of the Saturn was hit. Hard. Crumpling her to a point where no resusitation could be possible. The airbags kicked in, pouring a powdery substance into the air of the cabin making it hard for me to breathe. I forced the door open and climbed out of what was left of her with only one scratch on my left hand. The Red Saturn would drive no more. Having walked out of that wreckage with but a scratch, I swore I would never drive anything other than a Saturn again. My new Saturn is a sleek 2000 grey-bronze model. She calls herself simply Dink. I have no idea why. I don't question it. The night I purchased her I jokingly asked her what her name was, not expecting an answer.
This Saturn is more than just a cold-hearted military veteran. She has a personality, like a risk-taking cat burglar. She too has a sense of duty instilled in her, and also an unequalled pride in her work, but doesn't put up with the abuse I've given other cars before. This one lets me know when it's time to take her in to a mechanic. She has her ways of telling me, like having the windshield wipers start acting strange making it difficult for me to see on rainy days. I have no choice at times like that but to take her in. She's a trickster, and playful. She's on her best behavior when I have ladyfriends on the passenger side, as if she were a chaperone on the date helping things go well. When alone she's unpredictable and cocky, and if I ask her to stop on a dime she skids a little bit until we tap the curb, suggesting maybe I should just be a little bit more careful next time.
The conventions for car names are many and varied, and each car owner has their own way of doing it. Perhaps the best way is to just be attentive to the car's needs, get a feel for her personality, and decide on those qualifications.
But if you're real quiet, on a cold and brisk night, and ask real politely... sometimes you'll be surprised the answer you'll recieve.