A major theme of the story is how in-tune a driver had to be with their car to stay ahead of the inevitable breakdowns, and the struggles the migrants faced getting to California without having to abandon their vehicles. Was car technology at the time simply too primitive for a long trip, or was it just the quality of the card that migrants could afford/that were available in the Midwest?
Everything was against them.
Today it takes 5 days to drive across the usa 600miles a day. With little concern that your vehicle will break down on the 3000mile trip.
The highway system was terrible in the usa in 1930, in 1919 (president )Eisenhower took 62 days to cross the USA on the (best route possible) Lincoln Highway in 80 trucks with a group of men that had moble machine shops. Using lessor roads and with less support would have even taken longer.
In the 1930's The vehicles were poorly made by today's standards, they required huge maintenance, things like wheel bearing that never get changed today we're regular maintenance items. on top of that poor farmers may have delayed maintenance in an effort to save money.
Google Eisenhower's trip it is well recorded and well provisioned giving you the best possible idea of 1919 highway travel , I'm on moble and can't paste a link.