Usually, if you give the car a bit of time, they'll let you know what they want to be called :)

Each car has their own "personality" and things. Each has their own feel. When I bought Vixen (took me two weeks to get her name) I knew the car felt like a female personality. My last car was an Altima named Solo. Solo was definantly male. Part of it has to do with design and stuff of the actual car. The personality is usually a blend of your own and a bit of the cars melded into one.

Sheesh...I know I sound like a crazy person but hey! I grew up watching Knight Rider, Airwolf, A-Team and Dukes of Hazard.