He was still mad and talking bitterly to himself in an undertone while he drove--telling the new Ford what he thought of city rules and city ways, and driving it as no Ford was ever meant by its maker to be driven.
The country north of Barstow is not to be taken casually in the middle of a dark night, even by Casey Ryan and a Ford.

The roads, once you are well away from help, are all pretty much alike, and all bad.

And although the white, diamond-shaped signs of a beneficent automobile club are posted here and there, where wrong turnings are most likely to prove disastrous to travelers, Casey Ryan was in the mood to lick any man who pointed out a sign to him.

He did see one or two in spite of himself and gave a grunt of contempt.

So, where he should have turned to the east (his intention being to reach Nevada by way of Silver Lake) he continued traveling north and didn't know it.
Driving across the desert on a dark night is confusing to the most observant wayfarer.