Automatic and manual transmissions are both used to change a vehicle's gear ratio so that it can perform more effectively and efficiently. These systems work in very similar ways, with the primary difference between these two transmissions being the way in which the driver interacts with them. In addition to these two basic styles, it is also possible to find a semi-automatic transmission, which blends elements of both.
Cars need to change gear ratios to work properly. If a car is driven with a low gear ratio, it can only effectively function at low speeds, and acceleration would cause the engine to “redline,” or reach the point beyond which it cannot operate very well. High gear ratios are needed for high speeds, but a car can't be driven at a high gear ratio when it is going slowly. Hence, transmissions are needed to allow the gear ratio to be changed as needed.
In the case of a manual transmission, the driver of the car must change gears as he or she perceives a need to do so. Drivers rely on cues like the driving conditions and the tachometer to determine the best moment to change gears, and they change gears by engaging the clutch pedal, causing the gears in the car to disengage, and moving the gear shift to select a new gear ratio before disengaging the clutch so that the gears will re-engage. As all drivers who have learned to drive a car with a manual transmission know, this procedure can be challenging, and the learning curve on manual transmissions can be steep and very frustrating.
A car with an automatic transmission selects the correct gear for the driver, using a complex communications system which incorporates information about the speed of the car, whether the driver is accelerating or braking, and the revolutions per minute (RPM) of the engine. All of this work is done without the driver's participation: to go forward, the driver uses a lever to put the car in “drive,” and to go in reverse, the driver uses the “reverse” option. Automatics usually also have a “park” and “neutral” option, and some have overdrive for high speeds, along with low gears for special driving conditions like snow.
Semi-automatic transmissions blend these two systems. The driver is allowed to select the gear, as with a manual transmission, but gear selection is accomplished with a lever like that used in an automatic transmission. No clutch is involved, and the car will usually switch gears for the driver if he or she fails to do so and the engine appears to be in danger.
From the point of view of the driver, an automatic transmission is much easier to drive. It also requires more serious repairs if it fails, however, and a manual transmission tends to be more gas-efficient when driven by an experienced driver. Drivers who like to get more performance out of their cars may also prefer to work with a manual transmission.