Well, you can think of it in terms of corrected WPM, where if you are 98% accurate, you're spending (at least) 2% of your time correcting your mistakes. So it's possible to get up to (at least) 68wpm.
Of course I'm assuming that you're the kind that fixes your mistakes mid test. So more accuracy does mean more speed. But it's also the same as the regard as more practice means more speed anyway.
67wpm is actually quite fast. I think this is a good read:
http://smallbusines…I have reached 105wpm on this website before. And that's with correcting mistakes as they appear. I feel that practicing for speed and practicing for accuracy are one and the same. Since you can't type fast if you can't type accurately, that's what I think. I went for speed, and my accuracy is still around 98%, I should probably improve on that as well.