What is an automatic gearbox?

  • One of the earliest convenience features in modern cars
  • The latest gearboxes feature 10 speeds
  • Core technology has evolved over decades

An automatic gearbox is a type of transmission that can change gear ratios without any input from the driver.

Shifting without effort - the automatic gearbox

As automotive technology moved towards a standard layout of a clutch disengaged with a pedal, and a multi-speed gearbox, the obvious convenience solution was to find some way of automating the process of engaging drive and changing gear ratios.

While many solutions were proposed, the dominant technology adopted a fluid-filled torque convertor and multiple automated internal clutches to engage a system of mechanical gears. For the user, the gearlever became a means of selecting forward or reverse, and the clutch pedal disappeared.

In the UK and some other countries, the simplified driving technique resulted in a split in licence types; if you take your test in an automatic car (regardless of technology, this is essentially defined as a car without a clutch pedal) you will only be permitted to drive automatics from that point, and will need to take a new test to drive a manual car.

If you have moved to the UK from a country that doesn't differentiate, such as Canada, you can exchange your licence for a UK automatic one, or take a UK test to drive manuals.

How traditional automatic transmissions work

2015 GM Hydra-Matic 8L90

Inside the torque convertor, the input rotation of the engine spins the transmission fluid and impeller, which in turn spin the output to the gearbox.

This combination of components works to provide low-down torque for pulling away, and to allow the fluid to move slowly without overheating when the car is at a standstill, effectively disengaging drive.

Other technologies include dual-clutch systems like DSG, or the infinite-ratio CVT.

Automatic gearboxes for efficiency

Over time, additional features have been added to enhance efficiency. Torque convertor lock up engages the drive and gearbox with a direct clutch, ensuring fuel economy and refinement are comparable to a high-geared manual car.

TCI Automotive GM400 Clean Neutral Transbrake Valve Body

Gear selection - once determined by road speed, engine speed and hydraulic pressure through an ingenious system of valves (the 'brain' of an automatic gearbox is a surprisingly organic looking object) - is now electronically controlled.

Modern automatic gearboxes in cars can feature up to 10 forward ratios. with fast shifting and intelligent coasting. Rather than being slower and less economical than their manual counterparts, they are often the better option - unless you really want that last bit of control in a performance car.

2017 Chevrolet Camaro ZL1 10 speed auto transmission

Looking for more jargon-busting motoring meanings? Head over to our Parkers Car Glossary page and take a look at our other definitions