This issue of usage just came up in conversation with a friend, and it’s one of those, “I totally know the correct way to use these words, but am I for sure right?  I should look it up but I can never remember to,” type situations.

Upon thinking about it, it’s intuitively easy to figure out which word is correct in which context.  My friend and I landed on the idea that farther is used when dealing with measurable distances, and further is used when dealing with “mental” distances.  Just to be sure once and for all, I checked Garner’s:

farther; further. Both are comparative degrees of far, but they have undergone DIFFERENTIATION.  In the best usage, farther refers to physical distances, further to figurative distances—e.g.:

“After Popping in to say hello to Sue’s dad, we walked further [read farther] up Main Street to the Maritime Museum.”  Elise Ford, “A Pieces of the Rockwell,” Wash. Post, 14 Dec. 1994, at C9.

That was certainly easy enough, and now we can all rest easy.