Originally Posted by Az B

I've got a 16 - 18. There's a particularly mountainous ride I do a few times a year (5000ft in ~40 miles) It starts with a 7 mile climb, followed by a 5 mile downhill, followed by a pretty steep 4.5 mile climb. After that steep climb it's downhill for a while then mostly rolling with one more short climb. So I start in the 18 and move to the 16 after the second steep climb. The low gear makes the long climbs a little easier, and the higher gear is much better on the downhills and rollers.

Someday someone is going to invent a way to change gears while riding. Sort of a gear changulator or something.

Az