Our family has always had strict rules for naming cars - it has to alliterate with the car's make or name, and it has to reflect the cars nationality. Bonus points are awarded if there's a good gag involved, and sometimes the good gag overtakes the rules, because, when in doubt, go with the gag.
So, for example, a Ford could be Fred, but a Fiat would have to be Fiorello. A Honda Civic could have a C or S name, because of the sound, but the name would have to be Japanese, so Sumiko, or Sodaiko.
Lately, we've had PHEVs, so we had a Ford C-max, named Carmen (Carmen Electra, specifically), and we threw the rules out for the Kia Niro, and just called it Robert (the Niro).
Our recently delivered Gravity DE (#120) has been christened Gram. Admittedly, that's a six million fold understatement, but we had to go with a name that had some weight.