Take this from a girl that knows half a thing about cars. I don’t know even close to everything, but I’m certainly not one of those chicks that thinks I’m hardcore into cars just because I know how to drive a stick.
As someone said before, most girls are into IMAGE. A girl will be into your car if it is hot enough (to her) to get her noticed by other girls. The types of girls that think like this mostly only recognize cars like Corvettes, Mustangs, things that are small and sporty appearing, etc because these are the most common “expensive” cars. Riding in an expensive car makes other females jealous. Please read: I don’t agree with this, I’m just telling you how it is.
You/your car gets bonus points if it’s a convertible. Having the top down gives the female more opportunity to be noticed as being “that lucky girl” riding in the cool car by other females.
It’s all an image/popularity thing.
However, not ALL girls are like that 
But seriously, it takes less than 10 seconds to sort out the ones who really truely like your car, vs. those who are just trying to bullshit you to one-up their friends.