« Last post by mathieu on April 18, 2014, 12:11:09 pm »
If you have to ask this question, the best advice is: take 11-34.
As others have said, the difference between 32 and 34 is about half a gear change or -6% in speed at the same cadence. This doesn't sound much, but feels big in the lowest gear.
If you ride at 20 mph, the kinetic energy of your bike and body is about a factor of 20 higher than the energy input from one pedal rotation. If you stop pedalling for a moment the kinetic energy keeps you moving, speed drops slowly and as air resistance drops with the third power of speed, the speed drop is much less than linear with time.
But if you ride uphill with 3 mph, kinetic energy is only about half the energy input from each stroke and gravity weighs linear with speed (at constant gradient). Each pedal stroke has a sense of urgency and speed gets a sawtooth profile because the energy input is only substantial when the crank arms are near horizontal. This 'do-or-die' pounding of the pedals doesn't feel comfortable and doesn't look great. The more rotations per minute, the smoother and more efficient the pedalling and the less the strain on body (knees) and mind.
So why doesn't everybody opt for 34t? Well, there is a small weight penalty and the greater efficiency with faster pedalling stops at about 80 rpm. With a 22/32 combination and a 700-32c tires, at 80 rpm you advance about 2.0 m/sec (4.5 mph). At a gradient of 8 degrees, the altitude gain is 0.28 m/sec, which for a weight of 80 kg for rider+bike takes 220W (proportionally more if you are heavier or carrying an additional load). There are not many recreational cyclists around who can produce 220W power in steady-state, say over 20 minutes. Many will reach their limit at 175W steady power output. But those who can produce more power, are lighter or cycle lesser gradients, don't need 34t.
Slopes of 8 degrees is about the maximum you will see on the TransAm. Not in the Appalachians, but in the Ozarks. The Rockies and the Cascades are also less steep.