The definition of gender is being male or female. Male = of or denoting the s*x that produces gametes, especially spermatozoa, with which a female may be fertilized or inseminated to produce offspring.
I dont think cars can fertilise each other - would therefore have to say they don't have a gender - but I mean everything exists in the mind so if you want yours to be a girl then so be it.
I find this whole gender debate stupid and utterly ridiculous - we've become a society scared of saying anything in case of offending someone. I prefer to keep things simple - a male is a male if it looks like a male and a female is a female if it looks like a female.
If they want to identify themselves as something else, then they better let me know or I'll probably end up offending them all day long
Give it a few years and someone will probably start a trend of identifying themselves as a kangaroo.