The gears of the transmission and the differential are responsible for the speed and acceleration of a vehicle. A gear is identified by it's "gear ratio". 

For example the first gear of the Nissan 240SX 5-speed transmission comes with the ratio of 3.321:1. It means (when you are on the first setting) the  racing prop-shaft turns once for each 3.321 revolutions the engine makes. 

racing prop-shaft

Image Source : Google

First gear permits the engine to function at the speed range in which it has the greatest power, even when the vehicle is moving at a slow speed. This is the reason for an engine's transmission.

A short gear has the lowest ratio of gears, and a taller gear is one with a higher gear ratio. As an example the 3.321:1 gear is smaller than one with a 1.902:1 gear.

When racing, it is important to arrange your gears in a way that you are able to utilize the full power of your engine in every gear. That is why it's not necessary to have a 5-speed gear when you're racing in a low-speed course that you don't shift to the fifth gear. 

In this scenario you can install a lower final gear that will result in decreasing the difference between gears and permitting you to utilize 5th gear. To increase top speed, you could add a higher-end final gear. This will result in raising the amount of difference between the gears in your transmission.