i believe that there is an analogy here with naming horses, which perform the same function of basic transportation, and then some. This is different from a pet, since the horse almost qualifies as a "partner in crime".

i used to call my old beat-up car "Rosinante", Don Quizote's old beat-up horse.

as a "partner in crime", some people may also name the car as a person of the opposite sex.

in a multi-vehicle household, a name identifies quickly the vehicle being referred to.