Gatsby's car has two major themes behind it: first, it is a representation of his decadence and wealth; second, it is the physical object that leads directly to his downfall at the end.
Gatsby's car is described as follows:
It was a rich cream color, bright with nickel, swollen here and there in its monstrous length with triumphant hat-boxes and supper-boxes and tool-boxes, and terraced with a labyrinth of wind-shields that mirrored a dozen suns. Sitting down behind many layers of glass in a sort of green leather conservatory, we started to town.
(Fitzgerald, The Great Gatsby, mrbye.com)
The opulence of the car reflects Gatsby's own direction in life; because he has little emotional or ideological purpose, he instead reflects himself through his belongings. The "rich cream color" of the car is a symbol for his wealth; he has made his money himself instead of inheriting it, and displays it through his public image. The car's color is unique, instead of a classic red or black, and so demonstrates Gatsby's ego. Because the car is so ostentatious, people around can look at the car and see how rich Gatsby is; he wants to influence people to respect him for his wealth so he can court Daisy.
His car, which "everyone has seen," is also the cause of his death. Myrtle's death in a hit-and-run may have been covered up, or the impact lessened, if the car had not been so well-known. Wilson is able to prove to himself that Gatsby is at fault explicitly because of the car. In this fashion, the ostentatious car is both a reflection and the end of Gatsby's life.
It is interesting, indeed, that Gatsby's car, described in Chapter Four of The Great Gatsby as possessing
a rich cream color, bright with nickel,...and terraced with a labrinth of windshields that mirrored a dozen suns
takes on mythical proportions. As he and Nick ride to New York behind these layers of glass it is as though they are in a "green leather conservatory." Clearly, in this chapter, the car is a symbol of power and the protection it is perceived to provide, as well as an allusion to the story of Icarus, who tries to flee from Crete by flying with wings made by his father with feathers held together by wax. Ignoring the warnings of his father, Icarus flies too close to the sun, and the wax melts, sending him tragically to his death.
Likewise, Gatsby is not careful of danger, and the windshields that "mirrored a dozen suns" defeat him as his luxurious cream-colored automobile transforms into the yellow--symbolic of corruption--"death car" while Daisy "tumbled short of his dreams" and Gatsby's American dream crumbles into an illusion. Gatsby's car, therefore, is but another example of his confusion of material values for aesthetic and emotional values.