Transmission types explained

If you're in the market for a new car, the chances are you won't have a choice in the type of transmission it comes with. Most news cars are fitted with an automatic transmission as standard, with the possible exception of cheaper entry models, high-end sports machines and some 4x4s or light commercials. And for the most part, that's the way buyers prefer it.

Increasingly, however, there are a number of different transmission types that wear the 'automatic' label. Dual-clutch and continuously-variable transmissions are perhaps the most common, but each differs in its operation and complexity, and ultimately the experience offered to the driver.

To find out how each transmission differs, we take a brief look at the six most commonly fitted transmissions available today.

Manual (MT)
The oldest transmission type is that requiring 'manual' gear selection by the driver. A gearbox comprising a number of forward ratios (typically six) and single reverse ratio all selected via a lever usually mounted on the floor between the front seats. The spinning motion of the engine is regulated by a clutch actuated by the driver's (left) foot. Most modern manual transmissions allow out of sequence shifting (i.e. you may shift from third to fifth gear), and do not require rev-matching to downshift. Manual transmissions are preferred by driving enthusiasts for the level of control provided to the car's driving wheels.

Automatic (AT)
Now the most common transmission type sold in Australia, the automatic controls both the 'clutching' action between the engine and the transmission (usually via a torque converter) and the selection of forward ratios (usually six). Most automatic transmissions are hydraulically operated with electronic sensors governing shift timing and sequence. In some models, the use of electronic actuation can provide the driver with the ability to override the shift sequence manually, via the use of the selector lever or steering-wheel-mounted paddles. Automatic transmissions are typically favoured by urban and family drivers.

Continuously Variable (CVT)
The continuously variable transmission (or CVT) is a style of automatic transmission that most commonly uses a belt arranged between two pulleys to vary the drive ratio to the driven wheels. This process eliminates the need for the clutch and gears found in most other transmissions types. The CVT is favoured by manufacturers for its lower production cost, ability to achieve better fuel economy and its 'seamless' or uninterrupted transmission of torque between the engine and driving wheels. Like automatic transmissions, CVTs are favoured by urban and family drivers.

Dual-Clutch (DCT)
It's best to think of the dual-clutch transmission (DCT) as a manual transmission that changes gears for you, only much faster. Unlike an automatic transmission, the DCT uses a pair of gear sets arranged with the odd gears on one shaft and even gears on another (usually six or seven in total). Each is operated in sequence by its respective clutch (hence dual-clutch) with the benefit being an almost uninterrupted transmission of torque between the engine and the driving wheels. DCTs are favoured by enthusiast drivers who prefer the ability to shift 'manually' or 'automatically' as required, and is also becoming a mainstream choice for brands including those sold by the Volkswagen group.

Single-Clutch (SCT)
Like the dual-clutch transmission, the single-clutch transmission (SCT) can also be thought of as a manual transmission that shifts gears on your behalf. As the name suggests, however, the SCT uses only one clutch (most commonly a torque converter) to interrupt the transmission of torque from the engine to the driving wheels. The transmission's ratios (usually six or seven) are arranged sequentially and may only be shifted in numerical order. The SCT is preferred by manufacturers who require automated gear selection at a higher torque rating than can be offered by a standard automatic transmission. Like the dual-clutch transmission, this transmission is favoured by enthusiast drivers who prefer the ability to shift 'manually' or 'automatically' as required.

Automated Manual (AMT)
Also know as a robotised manual transmission, the AMT does not change gears automatically in the true sense of the word, instead relying on a series of sensors and actuators to execute gear changes as directed by the driver or computer. In the same way, the AMT also controls the operation of the clutch. Essentially, the AMT should be thought of as a manual transmission that does away with the clutch and lever. But, like the traditional manual gearbox, takes time to disengage and re-engage the delivery of torque from the engine to the driven wheels while the ratio is changed. A cheaper alternative to the traditional automatic, the AMT will suits buyers uncomfortable with the operation of a manual transmission but is generally too slow and 'clunky' to be considered a replacement for a regular automatic.