He's using at least a 54 front chain ring, as the solid Dura-Ace TT rings come no smaller than that. There's no doubt that he uses a smaller gear than those who use much slower cadence. The ability to turn 56x13 at 95RPM vs. 80RPM would put him many minutes ahead, rather than just seconds...

I hope I've explained well enough.

sometime in the last 1-2 years I read somewhere (maybe in his book) that he rides a 56 and that is what he starts his TT's on, meaning he starts out in his big ring and if I remember correctly a fairly high gear. I wouldnt be surprised if he runs an 11 because I believe Steve Larsen ran a 56/11 at Hawaii 2 years ago and that is when he smoked everyone on the bike.
Gettyup

http://www.lancearmstrong.com/...online2.nsf/html/FAQ:

"His usual rear gearing is 11-21 but sometimes he'll go 11-23 depending upon the difficulty - in training it's 12-25 and it's always a 53/39 up front."

The difference being 4.5 hours for 112 miles is under 25 mph. Armstrong's stage 19 TT in the tour was 63 minutes for 32 miles. That's 31 mph. 100 rpm for 54-12 is 30.6 mph.