Second best

If the dictionary lists both, then the hyphen is optional. I prefer using it in the adjective second-best.

AnonymousI know you should use the second largest car. no hyphen.

I don't know that. I would write "second-largest car" with the hyphen. What rule told you to omit it?

According to National Geographic (magazine) guidelines

Do not hyphenate:

g) ordinals with comparatives or superlatives:
second largest producer, third longest tunnel, first ever study, but first-grade potatoes, second-class citizens

The hyphen can help you to retain a distinction:

1. My second-best friend is Sylvie.

2. My second best friend was Igor. (My first was Lysette.)

AnonymousAccording to National Geographic (magazine) guidelines Do not hyphenate: g) ordinals with comparatives or superlatives: second largest producer, third longest tunnel, first ever study, but first-grade potatoes, second-class citizens

Thanks for the citation. Just further proof that punctuation such as hyphens are matters of style, and one s

I understand this distinction, but it seems that dictionaries use these forms interchangeably for the case of "second-best friend" (not second best friend).

Also, they use "second largest car" to mean the second-largest car, stating that no hyphen is needed.

This is where I am confused.

I think that many editors prefer common sense over hyphenation. That too is a style choice. Notice in addition that a superlative logically indicates a single entity.

Don't obsess over this. If you don't want to use the hyphen, don't. Just don't write "third-place finish" in one paragraph and "second longest tunnel" in the next. Be consistent.

Does this mean that if I use "the second-best car," I should use "the second-longest tunnel"?

Yes, indeed.