This conundrum comes with an easy distinction – if you can measure it, use “farther.” If the distance in question is more of a figurative issue, use “further.”
You’ve huffed and puffed every morning for the last two weeks, working on increasing the distance you’ve jogged each morning. In celebration, you jump for joy and cry out:
Or take the perennial favorite of summertime car travel. After 300 miles in the car, Junior is tired of watching “Toy Story 3” looping from the back of Dad’s headrest, sick of straws that won’t poke decently into his juice box, and with a huge whiny sigh, says:
“Are we almost there yet?”
Since this involves distance, you respond in soothing, grammatically correct fashion from your perch in the front:
“Only 500 miles farther and we’ll be there, son.”
For things that are a bit more vague, use “further.”
“We need to spend more time and money further studying this complex issue.”
Sometimes it gets tricky, though. Let’s say your book club is in the middle of reading War and Peace and you’re comparing notes with a club member, who glances pointedly at your bookmark, which isn’t stuck nearly as far along in the tome as hers is.
“Oh,” she says with a contemptuous sniff. “I see I’m further along in the book than you are.”
Sure, you could track your progress by pages read, so in this case, either “farther” or “further” would work.
As in, the farther you are away from her, the happier you’ll be…