Well, you see, there's no real spelling for a word that's not English.  What the English translations could come up with is a word that sounds like the Japanese original.  Usually, this is done by romanizing the word, but if you did that directly, what you actually end up with is: "Sefuiiro", doesn't look very good does it?  That's why people adopt the spelling "Cefiro" or "Cephiro" for it.  They are pronounced the same way, hence satisfying what it was made for: to sound like the Japanese original but still be readable by non Japanese speakers.
Yes, "Cephiro" is the "official" English translation that Media Blasters and Tokyopop (they released the Anime and Manga respectively) adopted but did you know that the word actually came from the Nissan Cefiro?  Check out the list of car names the names in MKR came from and you'll know.  Still not convinced? Go on any major search engine and look for "Nissan Cefiro", then search for "Nissan Cephiro" and see the difference you get.  If you do it on Google, it even tells you it's "Nissan Cefiro" you're looking for ^_~
So summing up, neither of us are wrong, we're just using different spellings of the same word (like how some people spell "meter" while others spell it "metre").