I find it terribly distracting when someone uses further for physical distance. I recently read a book by a friend who did that, referring to some guns firing further than others. It made me cringe, but I did keep reading.
On the other hand, I recently picked up a free book on kindle, wherein someone farthered their goals. That made me stop reading altogether.
Admittedly, that second instance could have been a typo, but given the poor grammar surrounding it, I don’t think so. I honestly suspect the author didn’t know the difference between the two words. That was just the straw that broke the camel’s back for me as a reader.
They’d both hit my linguistic pet peeves (I actually have one thing that bothers me more than this.) and that made it difficult to read their work. One was simply more clearly wrong.
So here’s the rule:
The quick and dirty tip is to use “farther” for physical distance and “further” for metaphorical, or figurative, distance. It’s easy to remember because “farther” has the word “far” in it, and“far” obviously relates to physical distance.
—Grammar Girl’s Quick and Dirty Tips
This is one of those rules that has a bit of flexibility because there’s a gray area between the two words. See the Ford ad above? Yep, it irritates me terribly because it’s trying to exploit that gray area to suggest that their cars not only take you farther, but they further….your goals or whatever else you want to put in there.
But it muddies the gray area further. People see the ad on television and decide that going ‘further’ is perfectly acceptable in English usage. After all, it’s on TV, so that means it’s right…
Anyhow, this is my current #1 on the pet peeve list.