Originally Posted by
revchuck
His original was nine-speed and would have been 6500. There were two ten-speed iterations, 6600 and 6700; the first 11-speed version was 6800 and the now-current version is 8000.
I put the Ultegra triple gears from my Trek Pilot 5.2 on my Madone 5.5 (replacing the stock compact double). I think my Ultegra is the 6600, but can't recall.
Doing a bit of Googling, I found this info showing the difference between the two:
ultegra 6600 vs 6700