I think clothes are more of a factor of how people treat you than what you drive. I remember walking into a car dealership one day to test drive a car--I was wearing a crappy T-shirt and jeans. The guy was rude and didn't give me any service at all. Then, I came back the next day wearing a shirt and tie from work and the same guy, not recognizing me obviously, was extremely attentive and polite. Old people, in general, tend to be more critical of what you wear, too. Maybe it's because when they grew up, everybody dressed nice (ever see a random street picture from the 40's?--everyone is in suits and fedoras, etc). A kid with cargo shorts and a t-shirt is a "hooligan" in many old folks' eyes. I'm generalizing, of course, but it's something I've noticed with many old people.
With cars, it's not as big an issue, because your car isn't around you most of the time---only when you're in it or standing near it. I think certain types of cars can correlate with certain types of people. For instance, I've noticed that guys who drive Camaros or Mustangs tend to be type-A egotistical jerks (especially when driving). Again, that's a generalization and not true for all Mustang drivers, but if you did a scientific study of personality types and matched them up with cars, I'm sure you would find some convincing data. You can, to some degree, tell something about a person's personality by what car they drive. However, you can't count on it. I may wish I was driving a Lexus, but if I can only afford a beat-up Geo, then your impression of me would be wrong.
So, in general, I think that people do form some preconceived notions about others based on superficial things like clothes and cars. It's probably wrong, and often leads to incorrect conclusions, but it's just human nature, I guess. Our brains function based on association, and this is just an extension of that principle.