Final answer:
The smallest increase in car ownership in the United States occurred between 1920 and 1930, as the automobile industry experienced significant growth in the 1920s and made car ownership more accessible to the average American.
Step-by-step explanation:
The smallest increase in car ownership in the United States occurred between 1920 and 1930. In the 1920s, the automobile industry experienced significant growth and mass production, making car ownership more accessible to the average American. This era saw the introduction of Henry Ford's Model T Ford, which played a crucial role in increasing car ownership. Prior to this period, car ownership was limited to the wealthy.