For better or for worse, at least over the last few decades, women have been attracted to guys with nice cars. Maybe it's a sign of the guy "having it together", or having the ability to provide, or at least showing the girl a good time.
So, if you want to impress girls, it makes sense to at least consider having an impressive car. If the size of your penis makes you feel lacking, you may way well try to improve your image with other means including automobiles.
But is the car an extension to the penis? Not hardly.
After all, a woman who thinks your a bad lover (due to your anatomy or your use of your anatomy) will STILL think you're a bad lover even if you get a better car.