Merriam-Webster adds that while further often is used to refer to physical distance, “I walked further than I had ever walked before,” the word farther can’t pinch hit for further for those non-distance usages. For example, you couldn’t use farther in “I must further investigate” or as a sentence modifier in “Further, there is evidence of foul play.”
Still, this dictionary is saying that there’s more overlap between these words than AP and Chicago allow. And you can use either word in “Joe drove farther/further than he had the day before.”
Note that the dictionary said that “currently” the words are diverging, which seems to fly in the face of Chicago’s, saying that a distinction between the words is “traditional.”