Cars play a very significant role in Gatsby in that they represent all that is fake and furious in this novel and with the characters. Nothing good ever revolves around them. Myrtle is killed by one. Tom's mistress's husband works at a garage (he's the one that ends up killing Gatsby). At the very beginning of the novel Fitzgerald has a completely drunk party-goer attempting to drive a car that is missing its steering wheel. Used symbollically, then, automobiles represent the hollowness of the status and wealth that was needed to purchase them. They have this wealth without the knowledge or ability to safely use them or drive them.
