By far + comparative

Question

Could someone please let me know whether "by far" can be used comparatively. Actually, almost all dictionary examples have used it in a superlative comparison, however I'm quite sure I have heard native speakers use it in comparative structures! Is it a common mistake by native speakers in informal speech or it is something grammatically natural?

Also, it is worth mentioning that I could not find any English grammar rule acknowledging this is wrong to use it in comparative constructions.

Examples:

1. Superlative: It is by far the best car in the world. (Which is perhaps the most common use of the term.)
   
2. Comparative: This restaurant is by far better than that one.

As a non-native one I cannot find anything wrong with it, but I have no clue why no dictionary has used it in comparative form?

Answer 1

The two "natural" sequences (most common first) for this "emphatic" comparative usage are...

1: This restaurant is far better than that one (by far the most common form)
2: This restaurant is better by far than that one (rather "literary" phrasing)

...in this context where ...is by far better than that... is at least "non-idiomatic" (but probably not actually "ungrammatical").

---

Note that [by] far doesn't directly relate to OP's "superlative / comparative" distinction, which is entirely governed by the choice between best and better. It's just that the syntax of how to use it differs.