Celer and vēlōx are often treated as synonymous. I feel certain that I learned the technical distinction between them once: that celer was potential speed, while vēlōx was actual speed. So Usain Bolt sitting on the couch would be celer (he's capable of moving very quickly), but not vēlōx (he isn't moving quickly at the moment). Conversely, stones being whipped around in a hurricane would be vēlōx (they're going really fast) but not celer (it's not their natural state).
However, neither Döderlein nor L&S makes any mention of this. Is my memory deceiving me? Or have others heard about this distinction as well?