What is the significance of the motifs of cars in relation to the theme of the book?

In chapter 4, we are given a description of Gatsby's car that begins with Nick's observation, "I'd seen it.  Everybody had seen it."  This tells us that Gatsby's car was one that stood out from the rest and the description Nick gives us, then, of the car shows how ostentatious it is.  It is big, bright, and it and it is meant to be noticed.  The car fits Gatsby because he wants to show people that he has money and he thinks that having the biggest and brightest car is one way to show that.  This is an example of class difference that is one of the major themes of the book.  Gatsby thinks that merely having money puts him into the same social class as Daisy, but it doesn't.  He is crass with his obvious display of wealth, whereas the Buchanans, with their sedate navy blue car, are much more understated.  The Buchanans don't need to show off their wealth in their car because they are comfortable with their wealth.  They've always had wealth and they wear it with much greater ease than does Gatsby to whom wealth is new.  He wears his wealth like someone with a new pair of shiny, ill-fitting shoes.  They look nice, but they are comfortable.  Another instance of a car playing a role in the story is the minor accident that occurs after one of Gatsby's parties in chapter 3.  This is meant to show the waste and the moral decline that occur in the story.   Of course, one of the biggest examples of how cars work into the theme of the story is when Daisy hits and kills Myrtle Wilson as she is driving home from New York City with Gatsby after the confrontation there.  This scene exemplifies the immorality of the story.  Not only is there a wasted life - Myrtle's - there is the immorality of irresponsibility because Daisy does not step forward and claim that she is the one who killed Myrtle.  She lets Gatsby assume responsibility.  She takes a life and then packs up and leaves.