As it has already been said before me, the economic growth of the 1920's was driven by more than just the automobile industry. There were a number of innovations which contributed greatly to it. Particularly important, for example, was the expansion of the electricity grid across the country, from urban centers into agrarian communities. The spread of electricity supported entire industries, as businesses produced all manner of household appliances which ran on the technology. At the same time, companies themselves were innovating, both in their use of advertising, as well as in their use of new methods of transaction (including, for example, the use of installment plans).
That being said, I'd agree that the automobile industry was one of the cornerstones of American business at the time. Ford's revolution in mass production had already taken in place, making the technology affordable to most people, with the result of giving people far more mobility, in their ability to travel from place to place. By the 1920's, Ford was facing increasing competition from rivals like General Motors and Chrysler.