Can someone explain the car scene?

I think some of it was to offer how a musician from an opposing genre felt about folk musically. It's unimpressive and uninspired or at least that's how Goodman felt. There's something about "authenticity" in there but I haven't formed a solid thought on that. I haven't seen it since it came out. At worst it was just set up for the Coen's to add more quirky characters.

Thematic metaphors used in the film are spectacular!

Early in the film, our broke and couch-surfing protagonist Llewyn Davis, crashes at a friend's house and accidentally allows their ginger house-cat (Ulysses) to escape. This wily feline disappears and reappears at various points throughout Llewyn’s journey, serving as a cryptic, adorable narrative device. Like many of the Coens’ symbols, the deeper meaning behind Llewyn’s furry friend is left up to interpretation. To me, Ulysses (not individually, but in form) was a metaphor for Davis's epic Homeric journey, his spirit's guide.

This car scene is used as a key motif in the middle of the film, when Davis hitches a ride with two other artists across to the city – a middle-aged and hardened jazzman, and a young actor/poet. The jazzman – a chewed-up cynic with a wound, a “habit” and a career (that makes ends meet); and the young actor – a self-deluding purist trapped in a humiliating symbiosis with the former – are both untenable, unbearable options for Davis in approach. His own grief, anger and loneliness is stuck in a Sisyphean loop – as was in the mythical anecdote of Sisyphus and Odysseus (Ulysses in Roman) – where the former keeps hauling a boulder up to the top of a mountain, only to have the boulder roll back down every time. The motif suggests to us how Davis may have angered "the gods" in this life or some imaginary past one. The jazzman implies as much during their car ride - Sisyphus' crime was "habitual dishonesty"; only this time it is Davis’ dishonesty with himself (the Gods imply).