An automatic gearbox is a type of car transmission that can change gear ratios without any input from the driver, as fitted to so-called automatic cars, also known as autos. There are several variations on the theme but the thing that links all of them – and makes them distinct from a manual gearbox – is that they don’t have a clutch pedal.
Automatic gearboxes are becoming increasingly popular in the UK, indeed there are many cars in which an automatic gearbox is fitted as standard to every model. Drivers like them because they’re easier than a manual, manufacturers like them because they’re often more efficient.
Every electric car currently available in the UK is an automatic, which is worth knowing as we’re encouraged to adopt the technology. Though, strictly speaking, most electric cars don’t actually have a gearbox at all, but we’ll come back to that.
Why your car needs a gearbox
The combustion process inside a petrol or diesel engine moves pistons up and down, which turns a crankshaft – that’s how the engine’s power and torque are generated. The crankshaft is connected to the gearbox, which is connected to the car’s driven wheels.
The crankshaft spins – or revs – too fast and with too little force to able to move a car by itself. The gearbox turns the crankshaft’s revs into a usable force. The gearbox itself is a metal housing containing cogs – or gears – of various sizes. Gears are also known as ratios or speeds – in Parkers’ new car reviews, we refer to a car as having, for instance, a six-speed gearbox.
First gear generates the most force to overcome the car’s inertia but can only be used at low speed. Higher gears generate proportionately less force but can be used at much higher speed.
How does a manual gearbox work?
The defining characteristic of a manual gearbox is that the driver has to change gear themselves by pressing the clutch pedal and moving the lever to the next gear.
When a car is ‘in gear’, two spinning cogs are meshed together. One is connected to the crankshaft via the input shaft, one – the gear itself – is connected to the wheels via the output shaft. The clutch separates the cogs and the lever moves the next gear into position. You release the clutch and thus you’ve changed gear.
You can change gear without the clutch, but you have to carefully match the crankshaft’s revs with the speed at which you’re going along the road. Get it wrong and you’ll hear deeply unpleasant graunching; the gear lever will be difficult to move, as well. Get it very wrong and you could strip the cogs of their teeth.
What’s different about an automatic gearbox?
The crucial difference between a manual and automatic gearbox is it that an automatic changes gear by itself. The driver doesn’t have to worry about a clutch pedal, however they still need to select the direction of travel – drive or reverse. There’s also neutral and park, and many automatics do allow the driver to take manual control of the gears, using either the lever or paddles on the steering wheel.
While automatic gearboxes are all the same in principle, there are there are several different types that work in completely different ways.
What’s the most common type of automatic transmission?
The most common – and oldest – type of automatic gearbox is the torque converter. It’s a rather complicated piece of kit but, in simple terms, it uses hydraulics to change gear. When a gear change is needed, the hydraulics release a clutch to separate the cogs, then moves the next gear into position.
Torque converters generally change gear very smoothly, if not particularly quickly. The hydraulics also need quite a lot of power, so they’re usually paired with larger engines. With a smaller engine, they can be quite inefficient. However, the latest torque converter automatics overcome the lack of efficiency with computer control and extra gears.
The gearbox’s computer generally picks the most efficient gear for any situation; the higher-numbered gears are specified to maximise fuel economy. For this reason, many automatic gearboxes now have eight speeds but some have as many as ten, including that in the Ford Ranger Raptor.
What other types of automatic gearbox are there?
A continuously variable transmission (CVT) doesn’t actually have any gears. Instead, it has two spinning cones, one linked the crankshaft, the other to the wheels. A belt loops around both cones and moves up and down them. That gives a virtually infinite number of gears which, in theory, maximises efficiency at any speed.
Driving a CVT-equipped car can be a strange experience. That’s because the crankshaft spins at a constant speed, so the engine noise often doesn’t relate to how fast you’re going. To get around that, some CVTs now have ‘steps’ that mimic gears. Japanese brand Toyota makes extensive use of CVTs, particularly in their hybrid models.
The other common type of automatic is the dual clutch transmission (DCT). Mechanically, the gearbox is much the same as a manual, albeit with electric solenoids that change the gears. The big difference is the pair of clutches – a manual just has one. The second clutch is primed to change into the next gear when needed, providing a quicker, smoother change. DCTs also tend to accelerate quicker than manuals for no loss of efficiency.
Volkswagen pioneered dual clutch tech with its DSG gearbox, which soon found it’s way into other VW Group cars. Many other manufacturers followed Volkswagen’s lead, as well, including Kia and Renault.
There are also cars that have an automated manual transmission (AMT). Like a DCT, it has a conventional manual gearbox with solenoids that change gear, but it has just the one clutch that’s operated automatically. You’ll find such a gearbox in the Fiat 500 Dualogic, among others. AMTs are generally quite efficient, but they can change gear rather clunkily.
Do electric cars have an automatic gearbox?
All electric cars are classified as automatics, however most technically don’t have a gearbox at all. When you select drive or reverse, what you’re actually doing is instructing the electric motor to spin forwards or backwards.
But there are some EVs that do have a gearbox, most notably the Porsche Taycan. It has a two-speed ‘box, in which second gear helps maintain strong acceleration at high speed, something EVs without a gearbox struggle to do.
Some manufacturers are experimenting with manual EVs, though they really only simulate the interaction you get with a car when changing gear yourself.
Do you need a specific license to drive an automatic?
Automatic cars are easier to drive than manuals and, for that reason, the UK has separate driving licenses for manual and automatic cars. It’s perfectly reasonable to take your test in an automatic – you may be physically unable to drive a manual, or just don’t want the hassle. Many instructors are encouraging their students to learn in an automatic, as well, though there are anecdotal reports they’re doing so primarily to get students through their driving test as quickly as possible.
Crucially, if you take your UK driving test in an automatic, you’re not legally allowed to drive a manual. If you subsequently want to drive a manual, you’ll have to retake your test in a suitable car. Conversely, if you take your test in a manual, you’re free and clear to drive automatics, as well.
There are a number of other countries that have the same licensing regime as the UK. If you got your license in a country that doesn’t make the distinction, then exchange it for a UK license, you’ll only be able to drive automatics. Again, if you want to drive manuals, you’ll have to take a UK driving test.
These days, automatics are so commonplace and so good that being limited to driving them alone isn’t the hardship it was once thought to be. We’re not just talking about new cars either; there’s a wide choice of excellent used automatic cars available, as well.
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