This question is for those who have done the tour before... I live in Florida where it is very flat and I use an 11-21 Durace 10 cassette. I don't know how steep are the hills we are going to be climbing but was I thinking of changing to 12-28 cassette. Is this too much or am I in the ballpark?
I did the ride last year with a 12-25, triple ring and had no problems. I used the small ring once for about a quarter mile. But, I'm from NC and hills are a part of all my rides. Don't fear them, learn to love them.
I assume that living in a flat area, you have only a double chain ring (no granny gear). I also assume that you don't get to do any training on hills. Overpasses don't simulate big hills. Headwinds are tough, but not quite the same. I'd go for the biggest cassette that works with your derailier (probably 12-27).
Use a 12-27 cassette if you have any doubts. A 12-25 cassette has 21-23-25 for its three lowest gears, while a 12-27 has 21-24-27. If you need more gearing on the hills, you will really regret it if you don't have them. In contrast, there is little disadvantage to having those extra gears if you don't need them.
The thing about weeklong tours is that your legs get progressively more tired from day to day as the week goes on. You might be fine with a 12-25 on the first day, but not on the 3rd or 4th. The route does get flatter as the week goes on, but you still will encounter plenty of rollers and moderate climbs in the middle of the week.
Bring the 12x27, Felix, it will help with the enjoyment. No need to hurry or worry, the hills ain't going anywhere, and there will always be another one just ahead, so they are fun, right? Spin down to your lowest gear, manage your body's workrate, and get over the top feelin' good and looking for something to eat! There's nothing like a little sweat on cool crisp NC morning in the country. Let's go now!
Some grades are steep, and lengthy. Last year there was a 2-mile climb on the first day that was a bit tough. If you haven't trained on some hils that are at least 1 mile long, I'd put the largest cogset in the back that your derailleur cage will accommodate. 12-28 is defiitely in the ballpark. If you can swap your small chainring from a 39 to a 34 or 36, that would also be in order.
The 12/27 should be all you will need. The major hills on the tour will be Sunday and Monday. From Eden on, you may encounter a few rolling hills along the way ( depending on the actual route we go to Mebane) but nothing to worry about. Remember, the course opens at 8:30 each morning and doesn't close to 5pm. This ain't no race. Take you time, pace yourself (other riders will do the same thing) and enjoy all the beauty that NC has to offer.