The cars become symbolic in Gatsby. Wilson works on cars while Tom and Gatsby drive them. These are symbols of wealth and power and they can be misused.
The scene following one of Gatsby's parties where a car is ruined in the driveway is the first example of a car being misused by its driver. The accident which kills Myrtle is another example of how power (represented by the car) can be used recklessly and can become dangerous.
One significant symbol in The Great Gatsby is the use of colors. Gatsby, of course, is yellow/gold, obviously a reference to new money. His car, his shirts, his tie, the splashes of color in his predominantly white house are all gold. Daisy is white. Refer to the first paragraphs in which we meet her, and everything on her and around her is white. Tom is red, the color of anger and rage. Again, our first impressions of him are infused by the color red. George is ashen, so much so that he actually blends into the walls of his garage. His wife Myrtle is a rather gaudy orange, suitable to her unsubtle, rather outrageous and loud personality. The colors match their temperaments as well as their status in the novel.
This novel abounds with symbols! Here are a few of the major ones!
Clothes and Books - represent the appearance of beauty and intelligence while simply being a facade. The shirts that Gatsby collects are not enough to lure Daisy to him while the pretentious books in his library are obviously unopened, unread.
Green Light - represents Gatsby's relationship with Daisy, at the simplest level. She resides across the bay in the "old money" part of town, close enough for him to see the light on her property, but far away enough to represent Gatsby's inability to claim her. Gatsby's "noveau riche" neighborhood does not intermingle with Daisy's, showing that a social hierarchy still exists, even for the rich. This light illustrates the "so close, yet so far" reality that is Gatsby's and Daisy's relationship.
Automobiles - represent extravagance and moral irresponsibility, as those who drive these modern, stylish cars do so while drunk and while angry, culminating in the tragedy which kills Myrtle.
Valley of Ashes - represents the waste and inequality of opportunity that happens in America. It is literally an industrial wasteland where the less fortunate live and exists just on the edge of the area dominated by the wealthy. The sad fate of its occupants represents reality that the wealthy do not like to acknowledge - that in some way, they may be contributing to the status of the less fortunate.