Despite all evidence pointing to the contrary, Hazel is certain that his car is of the utmost quality. This conviction brings about dramatic irony, as it's clear to the reader (and many of the other characters) that the car is falling apart. When Hazel visits a mechanic at the end of Chapter 6, the mechanic more or less tells Hazel this, so Hazel goes to another mechanic. The second mechanic hyperbolically sings the car's praises, making Hazel content and the reader skeptical:
At the other garage he went to, there was a man who said he could put the car in the best shape overnight, because it was such a good car to begin with, so well put together and with such good materials in it, and because, he added, he was the best mechanic in town, working in the best-equipped shop. Haze left it with him, certain that it was in honest hands.
The final sentence in the passage contributes valuable characterization. Although Hazel hasn't so far given the impression of being a likable character, his critical reticence has made it seem as though he's astute—especially because the reader encounters him alongside the talkative and foolish Enoch. When Hazel acquires his car, however, O'Connor begins to take this impression apart.
It's evident to the reader that the mechanic's statements about the car, his own skill, and his shop are hyperbole. All of these statements are so extreme that there's no doubt that he's lying in order to keep Hazel as a customer. When Hazel proves willing to trust these statements, the reader gains telling insight into his psyche. Hazel left the other mechanic because he told him something he didn't want to hear. He's content with this mechanic, on the other hand, because he tells Hazel what he wants to hear.
Hazel's ironic claims about his car continue as it continues to break down. In Chapter 7, he's once again forced to go to a garage. Even if the man who helps him this time, a one-armed man, remains silent, Hazel defensively insists that it's a "good car" and that it will take him "anywhere" he wants to go. To boost his ego, but perhaps also to mock him a little, Sabbath Hawks echoes this opinion with a simile: “It’s a grand auto [...] It goes as smooth as honey.”
Hazel's ironic certainty that the car is top-notch shows that, while he may seem much smarter than Enoch because of his brooding demeanor, he's just his own brand of foolish. Hazel thinks he's uncovered the truth, but he's constantly lying to himself. Because he sees the car a sort of extension of himself, he's unreceptive to opinions that run contrary to his ideas and plans. Given that he makes the car both his home and his escape from his surroundings, his unwillingness to listen to the honest mechanic sets him up for vulnerability later on. When the car meets its demise toward the end of the novel, he does as well.