Ground Control To Major Tom! The Transmission Guide: Everything You Need to Know

Amongst the mysteries of the universe, the inner workings of our automobiles are up there for many people.

We trust our cars to get us from A to B, but unless you’re mechanically minded, you don’t ask too many questions about how they do it!

Car Transmission

For most of us, automobile care doesn’t extend beyond putting air in the tires. Wheels, we can understand! But how they’re driven to turn when we put the key in the ignition… that’s another story.

But gaining a deeper understanding of your vehicle is valuable. Once you start down the road of mechanical know-how, you open up a whole world of car maintenance that can save you a fortune.

By understanding your car transmission, you’ll be able to troubleshoot and diagnose any issues on the road. You’ll have a head start when you drop into the garage, and probably get a better deal as mechanics know they can’t take you for a ride.

And if you drive stick, it might even make you a better driver. Quit grinding gears and steer yourself towards a smoother drive.

Before we strap in for the complete guide to your car’s transmission, let’s understand the glossary first so you would understand better the inner world of automobiles.

Transmission Glossary: Getting From A to Z

Clutch: The device in a manual transmission that disconnects the engine from the transmission.

Collar: The component connecting gears to the drive shaft in a transmission.

Driveshaft: The shaft transmitting torque out of the transmission to the differential.

Gear: A wheel with any number of teeth around the edge, used to adjust the ratio of speed between moving parts.

Layshaft: An intermediary shaft within the transmission, not directly connected to either the engine or the differential.

Planetary Gearset: The system of gears found within an automatic transmission.

Splines: Ridges along a shaft that enables the collar to move up and down between gears.

Tachometer: A dashboard instrument that indicates the rpm of an engine.

Torque Converter: The component in an automatic car that allows the engine to disconnect from the transmission so that the engine can work without turning the wheels.

What Does A Transmission Do?

The transmission connects the engine to the wheels – it’s a key component of every vehicle. It’s a system which is necessary to keep the engine from exceeding its maximum rpm and staying within the narrow range of optimum rpm for performance.

If you stick to automatics, your vehicle might not have a tachometer on the dashboard, but if you’re a stick shift fanatic this dial will be familiar as the rpm gauge.

This little dial is your window into the car’s rpm, and you probably recognize implicitly the relationship between your car’s transmission and revs: shifting up a gear will lower the revs, and if you’re over-revving the vehicle, it’s probably time to shift!

When we’re pulling off, it’s common to hear the engine over-revving as we pressure the clutch and pump the gas in first gear. But as you shift the transmission into second gear, you’re changing the gear ratio between the engine and wheels, getting away from the redline and towards your car’s optimal rpm.

The transmission also enables you to disconnect the engine from the drive wheels altogether – after all, there are times when we want the engine to be purring away without the wheels moving. By disconnecting the engine and wheels, the transmission enables the car to idle.

Low gears are vital for quick acceleration, whilst higher gears enable your car’s engine to run more efficiently when you’re cruising the highway.

The transmission is a crucial component, from idling to accelerating to cruising at constant speeds, it’s doing the work.

So that’s the gist of the transmission – your vehicle isn’t going to work without it. But how does a transmission enable the changing of gears?

And what about automatics – do they have transmissions too?

In fact, there are many kinds of transmissions in various vehicles. Let’s take a look at a simplified transmission to understand how shifting gears work before exploring the various transmissions you’ll find in vehicles on the market today.

A Simple (Manual) Transmission

A transmission is literally a system of gears, designed to mesh appropriately to adjust rotations. When you talk about changing gear, this isn’t a metaphor – you’re literally selecting a new toothed wheel, just like the gears you can see under the chain on a bike.

Of course, it’s a little more complicated than a bicycle, where the chain hops from one gear to another to adjust the ratio of rotation of the pedals and the wheels. But these toothed wheels are at the heart of a transmission. Let’s see how they work.

In the simple transmission below, the shaft from the engine turns the layshaft at the bottom of the diagram.

The layshaft connects to the car’s gears shown in blue- in the diagram, it’s a two-gear transmission, but most cars have five or six gears to select.

The blue gears are always turning – but they’re not always connected to the splined drive shaft which leads to the differential. It’s the process of changing gears through the gear selector fork that connects the chosen gear to the differential, determining the RPMs. When unselected, the gears freewheel on their bearings whilst the engine is on.

When you select a gear – moving into first gear from neutral, for example – you connect the gear to the drive shaft. The collar, attached to the drive shaft through the splines, slides along the shaft to the chosen gear and through teeth (called dog teeth) connects to the gear. Now the driveshaft is fixed to spin at the appropriate rpm.

When the engine is running, the layshaft is turning – and the blue gears are turning too. But until you select a gear and connect it to the driveshaft through the collar, the driveshaft and wheels remain stationary.

Similarly, if the wheels are turning, the drive shaft is spinning. But the shaft can turn inside of the blue gears if the collar is disconnected, allowing the engine to turn off and the layshaft and blue gears to remain stationary while the car coasts.

That’s about as simple as a transmission can get – and you won’t find this exact system inside any cars on today’s market. Let’s take a look at the different transmissions cars are equipped with within 2021.

The 5 Types Of Manual Transmissions

Although manual transmissions are increasingly rare across the United States – existing in less than one-sixth of new models sold in 2020 (1) – they’re still preferred by purists who believe that manual transmission offers unprecedented feel for a car.

With a manual transmission, the driver uses a clutch to disengage the engine from the transmission, enabling the change of gear. The clutch is propped between a flywheel and a pressure plate – pushing in on the clutch pedal releases the pressure plate, disengaging the transmission from the engine and letting the driver change gear.

But there is a multitude of manual transmissions out there – from old school to high-tech. Let’s take a look at all the manual transmissions on the market.

1. Dual Clutch

Dual-clutch transmissions use two clutches in the system, one operating the even-numbered gears (two, four, and six) and the other operating the odd gears (one, three, five, and sometimes a seventh gear).

Today’s dual-clutch transmissions are actually something of a hybrid between manual and automatic transmissions. Gears are changed through a shifter stick, but there’s no clutch pedal, usually familiar with manual transmissions. Instead, the clutch is operated by a computer system that automates the process.

These transmissions were historically popular but in 2021 are found in fewer vehicles – mainly German manufactured cars such as Volkswagen and Audi, claiming greater fuel efficiency than traditional 6-speed manual and automatics.

2. Unsynchronized

You won’t find unsynchronized transmissions in modern vehicles, but all the early manual transmissions were unsynchronized and they’re still used in some trucks today.

Also known as non-synchronous transmission, these systems were exceptionally durable, but entail frequent grinding of gears during shifting because of an absence of speed synchronization.

Even in unsynchronized transmissions, gear grinding can be avoided through float shifting (shifting without the clutch) or double-clutching: however, these techniques require proper training and skill or they can result in damage to the transmission.

3. Synchronized

Most modern cars utilize a synchronized transmission (also known as constant mesh) to remove the need for double-clutching and enable smooth gear changes, without that painful grinding sound.

Synchronized transmissions use pads to slow the gears down whilst maintaining a constant spin on the gears. If you’re driving a manual, it likely features a synchromesh, enabling a smooth connection with the collar’s teeth by slowing or speeding the relative speed of the required gear as you shift.

4. AMT

Standing for Automated Manual Transmission, this seeming contradiction in terms is a common transmission in big rigs.

Also known as a clutchless manual, AMTs use a computer to control the clutch whilst requiring manual shifting from the driver through the gear stick.

In passenger vehicles, AMTs have been increasingly replaced by DCTs but remain widespread in low cost cars across the world, and particularly in Africa, Indonesia, and Indian markets.

5. Single Clutch

Largely replaced by dual-clutch and synchro mesh systems which are smoother and can handle greater torque, single clutch transmissions utilize a computer to handle the shifting.

There are a variety of mechanisms used within single-clutch transmissions and clutch control and shifting can be either electronic, hydraulic, or a hybrid approach.

Automatic Transmissions

Automatic transmissions are fundamentally different from manual transmissions, even partly automated ones because the process that initiates the shifting happens within the transmission itself, rather than coming from an outside actor – whether that’s a computer or the driver themselves.

Although early automatic transmissions still utilized clutches, today’s modern automatic transmissions change gears through a torque converter.

Of course, automatics still have a shifter. But rather than changing gears, once the shifter is in drive, everything else happens automatically.

Utilizing an ingenious device called a planetary gearset, automatic transmissions enable all the gear ratios to be contained in one unit. Whereas a manual transmission requires the shifter to select between different sets of gears to create the ratios, the planetary gearset within an automatic transmission contains all these ratios ready to go.

The planetary gearset removes the need for driver input and the transmission shifts automatically through the gears!

Continuously Variable Transmissions

CVTs are automatic transmissions that vary the gear ratios by adjusting the diameters of two pulleys – a drive pulley and a driven pulley. A steel belt runs between the two pulleys, and the gear ratio is able to vary continuously, rather than in the stepped fashion in manual transmissions.

CVTs have traditionally been popular in small vehicles such as snowmobiles and ATVs, but they’re found in some modern-day hybrid passenger cars too.

Frequently Asked Questions About Car Transmission

Any questions? Here’s the FAQ to put your mind at rest.

1) What Does The Transmission Do?

The transmission is an intermediary component that moves power from the engine and transfers it to the wheels of the vehicle. It balances the gear ratio between the engine and the wheels, keeping the car’s rpm below the engine’s redline – that dangerous limit at which the engine might explode.

Through a series of gears, it enables the ratios to remain optimized, allowing you to drive efficiently and safely.

2) What’s The Difference Between A Manual And Automatic Transmission?

Manual cars have three pedals – a brake, an accelerator, and a clutch, whilst automatics have just two – the brake and the accelerator. In a manual, the third clutch pedal gives the driver control over connecting the transmission to the engine, and requires that they change gears themselves – manually!

In an automatic transmission, gear changes happen automatically. Thanks to the torque converter and planetary gearset, gears shift without driver input.

3) What’s Going On When I “Grind The Gears”?

Stick shift drivers might be familiar with a cringe-inducing grinding sound that occurs when you miss a gear. In fact, this sound doesn’t originate from your gears grinding, but from the collar’s dog teeth failing to connect with the gear within the transmission.

In a manual vehicle, when you change gears you connect a collar on the driveshaft to the chosen gear. That grinding sound is the collar bumping into the gear without connecting.

4) What Would Happen If I Put The Car Into Reverse While Moving Forward?

Ever felt a moment of dangerous curiosity whilst speeding down the highway – the temptation to shift the car into reverse, just to see what would happen?

Reverse gear is connected to a transmission by an idler gear, and thanks to the laws of physics, that means the gear is turning in the opposite direction to every other gear in the transmission. Makes sense, since it controls the reverse direction for the wheels, too.

If you tried to slip the car into reverse while the wheels were moving forward (and, concurrently, the gears in the transmission were moving forward) the collar that connects to the reverse gear would fail to mesh. It can’t catch the gear whilst it’s moving in the opposite direction.

It’s therefore mechanically impossible to hit reverse gear while the car’s moving forward. You’d hear a heck of a grinding – but nothing more.


You’ll find a transmission in every vehicle – without one, you’re going down a one-way street to engine overload.

But the kind of transmission under your hood? From synchromesh to CVT, there’s a lot that can be going on down there.

And even though manual transmissions are getting rarer, amongst a certain breed of motorhead there’s nothing that compares to the smooth shifting of gears as the engine roars.

Understanding your car’s transmission won’t just make you a better driver, but it’ll empower you in the garage to get the best deal. There’ll be no more mystery to the sound of gear grinding. Getting to know your vehicle is a journey that’s worth undertaking.

Science fiction writer Arthur C. Clarke famously said “Any sufficiently advanced technology is indistinguishable from magic.” The transmission is the witches cauldron: hubble, bubble, boil and gear ratios.

Robert Muñoz

I’m Robert, a US-based auto electrician, auto mechanic, trained engineer and fanatic about all things motor vehicle. After studying engineering in college I returned to my original passion - car mechanics - and I ran a garage for a number of years serving my local community. Through my garage, I got involved in numerous road safety campaigns in my local area until eventually, I decided to share what I've learned with the world. Know more about me... You can follow me on LinkedIn.

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