Here is what the American Heritage Dictionary has to say about the subject:
"Since the Middle English period many writers have used farther and further interchangeably. According to a relatively recent rule, however, farther should be reserved for physical distance and further for nonphysical, metaphorical advancement. Thus 74 percent of the Usage Panel prefers farther in the sentence "If you are planning to drive any farther than Ukiah, you'd better carry chains," and 64 percent prefers further in the sentence "We won't be able to answer these questions until we are further along in our research." In many cases, however, the distinction is not easy to draw. If we speak of a statement that is far from the truth, for example, we should also allow the use of farther in a sentence such as "Nothing could be farther from the truth." But "Nothing could be further from the truth" is so well established as to seem a fixed expression."
End of quote.
American Heritage Dictionary