Originally Posted by
dsaul
For going up hills, it doesn't matter too much. For road riding at higher speeds, most people have a desired cadence range. The next higher gear should fall into the range for efficiency.
For example, I like to be in the 93-105rpm range on a road ride. When I reach the 105rpm limit of my comfort zone and shift up to the next gear, I would like it to put me near my low limit of 93rpm. This allows me to keep producing a good amount of power without too much effort to get back into my most efficient cadence. If the next gear puts my cadence into the 80's, I have to put in a hard effort to get back up to my desired cadence and makes the ride less enjoyable.
dsual's explanation probably explains why it's good to have gears that are properly spaced pretty well. They don't have to be equal, Just close - perhaps 10 - 12.5% increases to match our most efficient cadence range.
It's like a car or boat engine, There is a range where they are most efficient. Same with people. Some might find a cadence of 80 - 90 rpm is best for cruising, Recreational cyclists like myself perhaps 70 - 80 rpm.