Automatic gearboxes are less common than manual gearboxes on vehicles in the United Kingdom and as a result drivers are often uncertain in which position the gear shift lever should be in any given set of circumstances. These notes are designed to assist the driver to decide.

If you understand the workings of the automatic gearbox you are more likely to make the right choice.

The majority of gear selectors are marked:-

For normal driving the selector may be left in the ‘Drive’ position and the transmission will automatically change up or down according to the load, speed and accelerator position. If it is necessary to manually change down to a lower gear, this may be done by moving the selector to the required position, but only if the vehicle is traveling within the speed range of that gear. (This facility must not be used excessively). When maximum acceleration is required the accelerator should be pushed to the fully open throttle position, overcoming any built-in resistance. This brings into operation the kickdown which causes an immediate down-shift into the correct gear for maximum acceleration. When the accelerator is released the gearbox will automatically change up again. Some gearboxes, especially computer controlled types, also have a kickdown that operates on partial depression of the accelerator. (i.e. Quickly depressing to the half open throttle position). It is because of the different methods in which gear changes occur that drivers are sometimes uncertain as to what action they should take to negotiate a hazard in the correct gear.

The Automatic Gearbox
	Let’s consider the make-up of the automatic transmission. In the majority of cases, this consists of a torque converter and a set of gears called a planetary or, epicyclic gear train. These are fitted to the vehicle in place of a conventional clutch and gearbox.
Torque converter....
	A torque converter consists primarily of an impeller, which is driven by the engine and a turbine that drives the input shaft to the gears. Each is bowled shaped and contains a number of compartments separated by vanes. These two bowls are placed face to face in a casing filled with oil and they are separated by a very narrow gap so that there is no rubbing contact between them. When the engine is idling, oil is flung out from the impeller, (just like air is forced through the blades of a table fan), and into the turbine. If the brakes are on, the force from the oil hitting the turbine blades is not sufficient to rotate the turbine. With the brakes off and as the driver increases the pressure on the accelerator, the pressure from the oil hitting the turbine increases and starts to turn it. As the turbine rotates, it transmits the drive through to the gearbox and the vehicle starts to move off. As the oil leaves the turbine it is re-circulated back to the impeller to continue the cycle. When the engine speeds up or slows down there is a certain amount of slip between the torque converter and the turbine as not all of the energy from the engine will be transferred one to the other. Some manufacturers’ now install a system which will mechanically lock the impeller to the turbine once a certain speed has been reached and in some of the gears. The torque converter, as the name implies, converts the torque or turning effort of the engine into the higher torque needed to drive the vehicle at low road speeds. An increase in torque has the same effect as changing to a lower gear, so a torque converter is also a gear reducer, acting like a set of gears before the engine’s drive reaches the gearbox. It is able to deliver this higher torque because there is a small vaned wheel, known as a reactor or stator, placed between the impeller and turbine. This wheel, depending upon the engine speed, directs oil along more favourable paths towards the impeller enabling it to give extra thrust to the turbine blades. At pull-away speeds, the torque converter can double the turning effort produced by the engine for the gearbox. As the engine speed increases, this 1:2 increase in turning effort is reduced down to 1:1 so that when holding a steady throttle opening and cruising, there is no difference in rotational speed and all the parts of the torque converter rotate at the same speed. (This would be when the lock up option on some vehicles will operate).

TYPICAL GEAR RATIOS
	Many automatic gearboxes generally have one less fewer forward gears than their manual counterpart because of this ability by the torque converter to vary the turning effort by acting as a gear with variable ratios between 1:1 and 1:2 There is a considerable overlap between each gear so that the third gear can give an overall ratio higher than the lowest ratio in the gear below. As the speed and power demanded to propel the vehicle changes the correct gear ratio will automatically be selected by the system so that staying in ‘DRIVE’ is not the same as keeping a manual car in top gear all of the time.
	TYPICAL GEAR RATIOS
	A Gearbox Ratio ~ 1:4 = 1 revolution out from 4 revolutions in

USING THE AUTOMATIC GEARBOX
	When stationary in traffic, even for several minutes, it is not necessary to move the shift lever into neutral. The torque converter absorbs the propulsion force from the engine. No wear is taking place , in fact more wear will occur if the driver continually changes from Drive to Neutral. When stationary, but in gear for any period, the hand brake should be applied and the foot kept clear of the accelerator. Whilst guidance may be given regarding the correct actions in certain circumstances, not all eventualities can be covered. Any gear change made by the driver should always be made at the ‘GEAR’ feature in the System of Car Control.

GENERAL
	In unusual circumstances when the gearbox is continually changing up and down, such as in heavy traffic queues, etc., select the lower of these two gears to prevent undue wear to the gearbox components. It is not necessary to either kickdown or change down manually to engage a lower gear for a hazard just because you normally would in a manual geared car. The torque converter and gear train are designed to select the correct ratio. On certain gearboxes the manual engagement of ‘2’ eliminates the kickdown facility. The gearbox will then prevent you getting out of a dangerous situation by rapid acceleration.
SNOWFLAKE
	This feature on some gearboxes reduces the torque at lower speeds, thus reducing the chances of wheel spin when accelerating from a stopped or slowly moving position. This system works by omitting the lower gear(s) and relying upon the slip through the torque converter to match engine and road speed.
SPORTS / ECONOMY
	In ‘Sports’ mode the gears are held in longer at higher speeds and loads. This would be the same as holding on to a lower gear, in a manual car, whilst accelerating hard. The gears will only change up once approaching the R.P.M. limit of the engine or, as the load through acceleration reduces (when you attain your speed and start to cruise). In this mode there is a greater tendency to kickdown on minor increases of accelerator depression. A ‘Sports’ option will normally be used if good progress is needed and may also be applied with lower gear holds too. An ‘Economy’ setting is the opposite of the ‘Sports’ option allowing the early selection of higher gears at the expense of performance acceleration.
CHOKE IN OPERATION (cold engine)
	On a newly started or cold engine there Is a tendency for the car to surge forward if the choke is operating and the tickover speed is higher than that normal. Because of this it is recommended that the foot brake be applied by the right foot before any gear is engaged whilst stationery.
WHICH FOOT?		12"?
	Normally only the right foot will be used on the brake and accelerator pedals. One condition however, where use of the left foot on the foot brake would be useful is while climbing a short sharp gradient and control is needed as the summit is reached. Use of the ‘other’ foot is very specialised and not necessary for normal driving. If the left foot is left hovering over the brake there will be a tendency for it to touch the pedal and illuminate the brake lights. This will not only dazzle the drivers behind, any warning of your use of the brakes will be lost as well. If you find it necessary to cover the brake with the other foot, in traffic, you’re too fast, too close!