Although I am a native speaker of English, I confess I have never been sure I understand the meaning of “if not” in sentences like “he is one of the fastest speakers, if not the fastest”. Does it mean (a) He is one of the fastest speakers, and maybe even the fastest, or (b) He is one of the fastest speakers, but not the fastest? Can it be ambiguous?
I would have never pondered that question…if not for your asking…
It means (a). He COULD be the fastest, but there is not enough data to know for sure.
It never means (b).
It means if he’s not one of the fastest runners on the world, then he’s the fastest.
Far be it from me to correct the knowledgeable Q.E.D., but I think a closer meaning would be – “He’s one of the fastest runners in the world. In fact, maybe he’s even the fastest.”
FBG’s formulation better expresses the nuance. But I would not call Q.E.D.'s formulation wrong.
I think it’s rather the reverse of QED’s formulation - if he isn’t the fastest speaker, he’s certainly among the top few.
I think its unbelievable that the meaning of “if not” is even being debated. Being 2 of the most simple and obvious words in the english language, it seems ridiculous, if not ludicris.
Thanks for the enlightenment. I still waver slightly, though. What about the sentence: “He can drive a car, if not a bus”? Could this not mean either (a) He can drive a car and maybe even a bus, or (b) He can drive a car but not a bus? Wouldn’t it depend on the context? Or is it always (a)?
It is always (A). “If not” is never used as a negative.
To use your first example:
He’s one of the fastest speakers in the world, if not the fastest.
To rewrite:
If he’s not the fastest speaker in the world, he’s one of the fastest.
Your second example is more ambiguous, but only because it is not a common usage for the phrase. Simply put, it doesn’t really make sense.
He can drive a car, if not a bus
To rewrite:
If he can not drive a bus, he can drive a car.
I hope that helps to clear things up. Personally I’ve only seen “if not” used as a comparison between one state and another.
ie. good, if not the best. bad, if not the worst. mediocre if not awful.
Ok, but when I typed in “if not the” into Google, one of the first 10 hits was this article: http://www.irvineworldnews.com/Astories/july3/golf.html
It is titled “He can drive the ball, if not the cart”. It is about a blind golfer who can play golf, but who cannot drive the golf cart.
I think it depends on whether the two items are subsets of each other. (I don’t think I have the ability to clarify that remark though.)
It seems to me that we have two disjunctive concepts being conveyed by different meanings of “if not” – one where it replaces “perhaps” and one where it replaces “though not.” Test these out in our example sentences – and note that the “one of the fastest” sentence is capable of either construction. It may mean that we affirm he is one of the fastest and choose not to take a stand on the possibility that he is the fastest, which we acknowledge may be true but leave open for debate, or it may mean that he is definitely ranked No. 2 or No. 3 or No. 4, but not No. 1, in terms of speed.
And I think you should be commended for a valuable contribution to the analysis, if not the precise definition sought! 
Wrong. Your misinterpretation is common, but the sentence really means:
He may not be the world’s fastest runner, but he is among the fastest.
Of course, this points to the improper usage of “fastest.”
The “two disjunctive concepts” interpretation of the construction is, I guess, what I use in practice. It just really bugs me since that it seems, in print at any rate, I often can’t figure out what the correct interpretation is supposed to be. In speech, I can hear the intonation, but in print, I would have to already know a lot about the topic to be able to choose, for example, between the interpretations “almost, but not quite the best” and “almost and maybe even the best” from context alone.
How could he be “the fastest”, without being “one of the fastest”? It would seem logically impossible.
I second the alternate reading “if he is not the single fastest, he’s certainly among the fastest.”
This usage can arise as a genuine/rhetorical afterthought, strengthened by parallelism to other equivalent phrasings, such as “He is one of the fastest -possibly the fastest,” many of which are equally natural, but seem a bit uneuphonious on examination.
I think most of what has been said is correct (the fastest speaker analysis). The above headline basically follows the same pattern, but may be confusing because it contains a play on the word “drive.” What the headline really means is, “We know he can drive the ball, perhaps he can drive a cart as well.” Essentially the same construction, but different from your “drive a car, drive a bus” example from earlier because in the car-bus sentence, “drive” had the same meaning in both clauses.
Hope that helps.
CrazyNMellow, just stick around a bit longer and you’ll begin to realize that this type of arguement is rather common here. 
Actually, I read it more as “we know he cannot drive the cart, but he can sure drive the ball”.
I do agree with you though on the more important point here that, in the case of this headline, the play on the word “drive” is the root of the confusion.
(Though I also find it quite unusual for a native speaker of English to be confused over this. Not that I’m doubting you (Lazy) or anything; it just seems unusual.)