Bike gears are the same thing as speeds. A gear or speed is one combination of a rear sprocket and a front chain ring connected by the chain. If a bike has three chain rings on the front and six sprockets on the rear wheel, you have three times six, or 18, combinations of chain positions. Operated by the shifter levers, the derailleurs on the front and/or rear of the bike change the position of the chain.
Effects
Whether your bike has one gear or 27, the number of teeth on the front chain ring and the number of teeth on the rear sprocket affect the difficulty of your ride and how far you travel. In one rotation of the pedals, the chain can move only as far as there are teeth on the front chain ring. If your chain is on the 28-tooth chain ring, your bike won't go as far in one full rotation as it would with the chain on the 50-tooth chain ring. Pedaling is harder when the chain is on a larger front chain ring, but on the rear sprockets, the larger the sprocket, the easier the gear. Note that some gear combinations give the same pedaling difficulty. For example, a bike in a 14-tooth rear gear and a 39-tooth front gear has the same resistance as one with a 19-tooth rear and 53-tooth front gear, assuming the bikes have equal crank lengths and wheel sizes.
Misconceptions
A bike with more gears doesn't necessarily give you significantly more speed options. Competitive cyclist and bicycling coach Levi Bloom states that you shouldn't use gear combinations that make the chain cross diagonally between front and rear cogs that are both large or small. This crossing wears out the chain faster and can cause derailleur problems. The more gears on a bike, the more gear combinations that are unusable, and the more gear combinations that are functionally equivalent, as noted previously. What's more important on any multi-speed bike is a good range of easy, medium and challenging gears for a variety of terrain, which you can have with a 10-speed bike or a 21-speed bike.
Considerations
If you choose a gear that is too difficult for the conditions, you'll pedal slower than is ideal. This builds muscle mass, but wastes your energy and gives you a higher risk of muscle strain and joint damage in the knees and hips, according to the late bicycle mechanic Sheldon Brown. On the other hand, if you choose a gear that is too easy, you pedal too fast and tire out faster. Though every cyclist's preference for pedaling speed and pedal resistance is different, fast and easy pedaling is the best exercise for road riders, because it tones the muscles without straining them.
Warning
Your chain must be at least long enough to wrap around both the largest rear gear and the largest front chain ring, with one inch additional length, when it is not threaded through either derailleur. Making your chain too long or too short can cause your chain to fall off the gears or to get stuck trying to shift between them.
Potential
Michael Bluejay, author of the BicycleUniverse website, notes that you don't need to get a new bike if you want bigger or smaller chain rings or sprockets. Any bike repair shop can help you choose new chain rings or sprockets and even install them for you.
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