The gear ratio of your transmission, timing belt/chain and even your analog clock are what is responsible for rotational movement and the speed each piece or part achieves. Setting the correct gear ratio is vital, especially in the automotive industry. The wrong gear ratio will rob you of power, performance and even keep your vehicle from running at all, in the case of a timing belt of chain.
If you've ever seen two gears working together, one turning the other as the teeth of the two gears mesh, you've seen a perfect example of gear ratio. This complex sounding conundrum is nothing more complex and complicated than the teeth of two gears meshing as they turn. It can also mean two sprockets connected with a chain or two pulleys with a drive belt. The best example of the sprockets and chain combination is probably the standard timing chain. This vital piece of engineering is the driving force behind most modern vehicles, though timing belts are still used by some manufacturers.
A correct gear ratio is the driving force behind anything that contains rotational motion. Engines, transmissions, clocks and even windup toys use gears with the correct gear ratio to produce the motion needed to turn whatever needs to be turned. Whether it's belts and drive shafts or tiny plastic axles and wheels, the correct gear ratio is incredibly important. One of the best examples of getting the correct gear ratio would be replacing a timing belt. If the proper teeth are not selected when putting the new timing belt back on, the vehicle will be out of time. In short, it will run either very poorly, or not at all. That's because the pulleys and gears must meet at exactly the right point for the rotation to match. If the rotational speed of the gears or pulleys doesn't match then you have the incorrect gear ratio.
Gear ratio is also used to increase the speed of gears and pulleys. If you have a large gear turning a smaller gear, the gear ratio will increase the rotational speed of the small gear or pulley, dramatically. A gear ratio is written as any mathematical ratio: 2:1, 3:2, etc. In an example, if the large pulley rotated once per every two revolutions of the small pulley, you would have a gear ratio of 2:1. Gear ratio is an observable factor, as well; look under your hood sometime, or inside a clock or anything else containing gears; you'll see that larger pulleys and gears usually turn more slowly than their smaller counterparts. This knowledge is used to create high speeds within engines and transmissions.
Gear ratio and teeth on the gears are inextricably related. If it weren't for the teeth on the gears, slight differences in circumference and other manufacturing inconsistencies would lead to an incorrect gear ratio inevitably. Since the majority of gears use teeth, those inconsistencies don't matter; the teeth make up the difference and provide for a lack of slippage. Pulleys, on the other hand, are frequently the same size and have a rubberized, or non-slip, outer covering. This combines with the autotensioner to keep the belt firmly seated around the pulley, rather than dangling down below the vehicle.
Gear ratio within a transmission is incredibly important. Gear ratio is what's responsible for your vehicle's acceleration and top speed. Both wide and close gear ratios have benefits that are inherent to that type, though most modern transmissions do a good job of running the middle ground between these two extremes.