The demand curve represents the total amount of consumers of a good or service, in this case, cars. As consumption per car increases, the demand curve shifts to the right on the graph. When consumption decreases, the demand curve shifts to the left.
Shifts together are associated with aggregate economic events. For example, a significant drop in gasoline prices would increase demand for the car, causing the demand curve to shift to the right.
However, if automakers announced that an even more economical electric car would be launched and popularized, consumers would decrease fuel consumption car by pulling the left-leaning demand curve.