The discussion Holden has about the difference between men owning a car or a horse occurs when he is out with Sally Hayes and he is opening up to her and explaining about all the things that he finds wrong with society. He is trying to explain to Sally that people are never satisfied with their material possessions, this makes his point that the world is full of phony people.
"Take most people, they're crazy about cars. They worry if they get a little scratch on them, and they're always talking about how many miles they get to a gallon, and if they get a brand new car already they start thinking about trading it in for one that's even newer. I'd rather have a goddamn horse. A horse is at least human, for God's sake." (Salinger (p.131)
Holden is trying to convince Sally of just how difficult it has been for him at Pencey Prep with all the phonies that he had to deal with, and how there is no sincerity in the world at all, except for her.
"When they take a break from skating, he pours out to her the depression he feels about the meaninglessness of his life. His impulsive solution is that they run off together to Massachusetts and Vermont. Sally is the first person with whom he has shared these personal thoughts, yet he knows that she is incapable of understanding what he is talking about. Finding no empathy, he becomes angry and insults her. Holden really is beginning to sound like a “madman.”
He uses the example of how little people value their cars to try to make the point about the character of the boys that he went to school with, he says:
"You ought to go to a boy's school sometime. Try it sometime, It's full of phonies and all you do is study so that you can learn enough to be smart enough to be able to buy a goddamn Cadillac some day." (Salinger)
Sally does not accept his logic or line of thinking and insists that people go to school for more reasons than that. But Holden is not really listening, he has already made up his mind. He gets a crazy idea in his head about leaving the civilized world behind and living a rustic life in a log cabin in the country. He wants to separate himself from all the demands that society makes on him, he believes that if he keeps running away from his responsibilities that he will truly find happiness.
Sally does not like the idea of running away and she rejects him saying that he is crazy, this causes Holden to get very angry at her and he insults her, she wants nothing more to do with him and leaves by herself to go home.