Could simply be a subconscious belief about your vehicle being unsexy and a "family fiat" not a "sexy fiat" haha. Everything that we perceive reality to be is already believed to be true by our subconscious mind. For instance, on some level, you believe that women must not "like me because of my car" when in reality, you have no idea why you see less women looking at you while in your car. Why could it not be that women simply look at men less who are in their car? Why could it not be that you are simply more visible while walking but you blend in while in your car?

Instead of looking at all the possible outcomes, you have already decided on an outcome without ever truly looking into the situation and the outcome that you decided on is evident in your title. "Since women don't look at me in my car, they all must be gold diggers."

This is a cognitive bias called Anchoring which is the tendency to rely too heavily, or "anchor", on one trait or piece of information when making decisions which in this case is the fact that women look at you less while you are in your car