Convex mirrors are used in car rear view mirrors as they form a smaller image of the object. Convex mirror can therefore be used to view a much larger area than it would be possible with concave mirror.
Concave mirror on the other hand forms magnified and inverted image of the object. This would form an image of a smaller area and would also inverted which would be hard to understand.
Thus, convex mirrors are used in car rear view mirror.