In a car with manual transmission (M/T) the driver decides when to change gear and does so by pressing the clutch pedal down and then selecting the desired gear by moving the gear stick.
In a car with automatic transmission (A/T) the gear change is made automatically, hence there is no clutch or gear stick. When the transmission is put into Drive (D) the car will automatically select the correct gear according to the load on the engine and the road speed. Usually, the A/T will change to a higher gear as the road speed increase, and to a lower gear as it falls.
The A/T will also change down to a lower gear when going uphill, as the load on the engine increases.
How to use the gear selector?
Nearly all automatic have a gear selector, which will usually include:
Park ( P )
This locks the transmission, restricting the vehicle from moving in any direction. This should only be selected when the vehicle is stationary (full stop).
A car should be allowed to come to a complete stop before setting the transmission into park to prevent damage. Usually, Park § is one of only two selections in which the car’s engine can be started, the other being Neutral (N).
Reverse ( R )
This engages the reverse gear, permitting the vehicle to be driven backward.
To select reverse in most transmissions, the driver must come to a complete stop, depress the shift lock button (or move the shift lever toward the driver in a column shifter, or move the shifter sideways along a notched channel in a console shifter) and select reverse.
Never shift into reverse while the vehicle is moving. Not coming to a complete stop may cause severe damage to the transmission.
Neutral/No Gear ( N )
This is the same as Neutral on M/T gearbox. This should only be selected when you have no need to control the speed of your car, not when driving regularly. Examples of this include when waiting for the green light in traffic, when idling parked for a short time, or when being pushed/towed.
This disengages the transmission from the driven wheels, allowing the vehicle to coast freely under its own weight and gain momentum without the motive force from the engine.
Freewheeling or coasting long distances should be avoided as the transmission’s lubrication pump is driven by non-idle engine RPMs. Coasting to a stop for short distances (e.g. towards stop light or toll gate) should be fine.
Drive ( D )
Select when driving forward. This is the normal driving position for the best fuel economy.
This allows the transmission to engage the full range of available forward gears. The number of gears depends on the model ranging from 4-speed to 9-speed (2013).
Third Gear (3) / D with Overdrive OFF
This limits the transmission to the first three gears, or sometimes locks the transmission in third gear. This can be used to climb or going down hill. This gear is also recommended while towing a trailer.
On models with Overdrive (O/D) switch, the “O/D OFF” indicator light should come on when you push the switch.
Second (2 or S)
This limits the transmission to the first two gears, or sometimes locks the transmission in second gear. This can be used to drive in adverse conditions such as snow and ice, as well as climbing or going down hill in winter, and for stronger engine braking effect.
First (1 or L [Low])
This locks the transmission in first gear only. This can be used during the winter season, for towing, or for downhill driving with maximum engine braking effect.
This is not intended for use under extended or normal driving conditions and results in lower fuel economy.