Hello from a newbie (after years off a bicycle). I've recently obtained a GT ZR 5.0 with Ultegra 6600 derailleurs (I can see the name on the rear, but not specifically on the front one, but I'll assume they came as a pair - bad assumption?). I'm looking to use the bike on some longer fund-raising rides in the range of 50 to 100 miles, but have found the current setup to be less than optimal for me since I'm not strong enough to really use the last two or three high gears. I did a 42 miler a couple of weeks ago on a heavier bike and took 3 1/2 hours with a friend, but I had lots of energy left at the end of the ride. Thus, I believe I can handle 15 mph for at least 4 hours on the ZR.
Here's my questions:
I'm looking at changing the chainring from 34T/50T to 34T/40T, to obtain a more usable range (it was originally set up by a very competitive tri-athlete) for the relative flats (minor hills) and some half-step gearing, but I have concerns that the FD can't handle that small a variance. Will the Ultegra 6600 FD still work? Are there any other concerns I need to be aware of before buying the 40T?