The question as asked isn't easy to answer but a good guess would be 17% and 80%. It would be fair to say that 5 times as many people have cars now compared to 1925. We know that 17.5 million automobiles were registered in 1925. read more
By 1920 there were 7.5 million cars and trucks in the United States. The automobile industy was promoting the building of highways and the use of automobiles was beginning to have an affect on railroads. read more
This is not to say that cars would not be a commonplace sight in cities - San Francisco, for example, had 461,800 vehicles registered in 1930, a rate of one per household, and as early as 1927 54% of families in cities over 100,000 had an auto. read more