The gears in the transmission and the differential control the acceleration and top speed of a car. A gear is identified using its “gear ratio”. For instance, the first gear in the Nissan 240SX 5-speed transmission has a gear ratio of 3.321:1. This means that (when the transmission is in first gear) the driveshaft rotates one time for every 3.321 rotations of the engine. First gear allows the engine to operate in the RPM range where it makes the most power even when the car is moving slowly. This is the purpose of a transmission.
A short gear is one with a low gear ratio, and a tall gear is one with a high gear ratio. For example, a 3.321:1 gear is shorter than a 1.902:1 gear.
In racing, you want to set up your gears so that you can use the full potential of your engine in each gear. In other words, it is not useful to have a 5-speed transmission if you’re driving on a relatively low-speed track where you never shift up to fifth gear. In this case, you could install a shorter final gear, which will have the effect of shrinking the difference between gears and allowing you to use the fifth gear. The opposite problem is when you run out of gears (reach the RPM limit in fifth gear). To allow for more top speed, you can install a taller final gear, which will have the effect of increasing the difference between your transmission’s gears. Before choosing a new final gear, you can use the Top Speed Calculator at http://www.240edge.com/performance/top-speed-calc.html to find what the top speed of your car will be with the new gear ratio. It is also possible to change the individual gears in your transmission, but this is not practical. It is usually only done by professional race teams.
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