How they differ, how they work, and which one is the best
If you are under the impression that there are only two types of transmission for a car - namely a manual and automatic transmission - you aren't alone. And, while there is much more to it than simply that, understanding what a transmission is will help to make these distinctions a little easier. In terms of a definition, a transmission is the link between the engine in your vehicle and the wheels. It is a crucial part of the drivetrain that transmits torque to the wheels via the differential, which splits the power between the wheels and is, in turn, connected to the driveshafts and wheels.
This torque is sent to the wheels using the appropriate gear ratio, which is selected either manually, or automatically. Manually selecting the desired gear requires depressing a clutch pedal to disengage the connection between the engine and the wheels in order to change gears. Depending on the transmission, there can be any number of gears: as few as five and as many as ten. Some transmissions have an infinite number of gears, which you'll read more about below.
The lower gear ratios provide plenty of wheel torque to climb steep grades and do hard work - simply put, more power, but less speed. The higher gear ratios result in far less wheel torque, but gives the vehicle the "legs" for economical cruising on the highway - more speed. Any transmission connected to an internal-combustion engine, whether a manual or an automatic, works on this basic premise. So, how do the different types work, and if you have to choose between manual vs automatic transmission, which is best?
The standard manual stick-shift transmission is the least popular of all car transmission types available in the US, and leaves all the work to the driver. Here, the driver must manually move a lever through an H-gate to select the gear they want while depressing a pedal that operates a single clutch to disconnect and reconnect the drive. This causes a momentary pause in acceleration, which some feel is reason enough to believe an automatic transmission is better than a manual gearbox. While this may mean some manuals don't feel as smooth or swift in between each shift, many enthusiasts prefer the option to be in charge of gear selection.
Most modern manual transmissions are of the constant-mesh type, with the shift lever operated by the driver connected to shift forks that move around dog clutches to engage the different ratios. To prevent the crunching associated with gear changes on old-style unsynchronized gearboxes, synchronization rings or cones are used. The friction clutch is a lossless, direct drive to the wheels.
Manual transmission cars are as old as the motorcar itself, but does that mean a manual transmission is better than an automatic? Here are some advantages and disadvantages to a stick-shift setup:
When we talk about the traditional automatic, we refer to one specific type that many people still call the 'slushbox'. As the word 'automatic' suggests, this transmission does not need the driver's input to change gears, and instead makes use of hydraulic power to shift; the required gear is selected based on feedback from sensors that determine how much power is needed. Standard automatic gearboxes comprise a planetary gearset that make use of a torque converter, which takes over the job of a standard clutch found on a manual transmission. Forty years ago, these transmissions had only three or four gears, but today they can have as many as ten, and have become very advanced.
Over the years there have been numerous trade names for this transmission type, including GM's Hydramatic, Audi's Tiptronic, and BMW's Steptronic, as used in the BMW X2. In fact, this durable design is used on many trucks and SUVs, especially those with high tow ratings. This type of transmission's default mode is automatic, but many models offer a manual mode to give the driver some control and mimic a sportier and more involved feel. In some cars, this manual mode can emulate a manual transmission, with the driver in control of the gearshifts - but without a clutch pedal - using either the gearshifter or paddles behind the steering wheel, to upshift or change down.
This is the most common transmission type in the US. What are its pros and cons?
The CVT is a different type of automatic transmission with no gears at all. Instead, it uses a special belt or chain that runs between two pulleys, shaped liked cones, to continuously vary gear rations, just as the name suggests. As the belt or chain moves up one cone, it moves down the other, effectively creating an infinite number of gear ratios to always keep the engine in its most efficient rev range. There may be a manual mode to provide shifting between certain preset positions on the cones to create the feel of a traditional planetary automatic gearbox.
More and more new cars are equipped with a continuously variable transmission, even larger sedans such as the Honda Accord, and has become especially common on hybrid cars. The benefits are many, but there are some disadvantages, too:
Dual-clutch transmissions (DCTs) are like two normal manual transmissions with two clutches, but integrated into the same unit to work as one transmission. One clutch is connected to the odd gears and the second to the even gears. The vehicle moves off with first gear engaged on one clutch and a preselector already engages second gear on the other, open clutch. The gearshift is nothing more than the first clutch opening and the second closing. Once in second gear, third gear is preselected again on the first clutch, awaiting its turn, and so forth. But, like an automatic, it's not the driver that shifts gears - here, shifts are initiated by an electronic control module. Popular DCT's include the Direct Shift Gearbox, or DSG, from the Volkswagen Group, as used on the Golf, and Porsche's PDK setup.
It all sounds like the perfect solution. Does this make a DCT the best automatic transmission?
The traditional automatic gearbox, also known as a slushbox, is still the most common in the USA. By and large, a good automatic that gets an oil change every few years can last hundreds of thousands of miles. A good manual gearbox can last even longer. However, today, just a fraction of cars sold in the US have manual transmissions; in fact, it was already down to 2.4 percent in 2016. Still, manual is better than automatic when it comes to driver engagement for the true gearhead. A CVT is cheaper to manufacture than a normal automatic and very fuel-efficient, so they're a quite popular automatic option for people who pursue ultimate gas mileage. Repairs are expensive, not all specialists can perform them, and some have to be replaced altogether. Twin-clutch gearboxes are the most complicated and expensive automatics. They offer great economy and performance and are best enjoyed in sports cars, where their rapid shifts make for very rewarding motoring. But they are expensive to maintain and to put right when they go wrong. Automated manual gearboxes are the cheapest in the automatic range, requiring only a few controls, switches, and servos to be added to a normal manual gearbox. They are a great auto option if the only measure is cost and convenience. However, jerkier shifts on some have caused them to fall out of favor.
Any transmission is compromised in some way, each with its own pros and cons, as you've seen above. So it boils down to what you prefer. The manual transmission is a dying breed in the US and has really become the go-to option for true enthusiasts who still want to do the shifting themselves. This is why some of the best driver's cars, such as the Ford Mustang, Dodge Challenger and Chev Camaro are available with a manual transmission. The CVT is the most efficient automatic transmission, but it doesn't feel and sound as good; cars like the Honda Accord and Toyota Corolla have this setup.
Dual-clutch transmissions are excellent for sports cars but can be a little bit of overkill for normal cars, especially considering their maintenance and repair costs. Performance-focused vehicles like the Mercedes-AMG CLA 45 and the Audi R8 all have a DCT equipped. Lastly, the trusty old torque-converter planetary gearbox that Americans have loved for decades, with the latest designs such as the ZF 8HP and GM/Ford's joint-venture 10-speed going to show there is still plenty of sportiness, economy, and durability to be had from a modern automatic.
The best option is to find something compact and easy to drive to practice on, so you can devote your full attention to learning the new skills. Easy-to-pilot cars include are the Chevrolet Spark, Hyundai Accent, and Mazda 3.
Most manufacturers recommend that you replace your automatic transmission oil every 45,000 to 60,000 miles. This is especially important for the more sensitive DCTs and CVTs, and if you tow a lot. Check your owner's manual and if it doesn't give specific instructions, err on the side of caution. This is something you should definitely keep an eye on if you are buying a used car. Be sure to check out our Car Maintenance article for more information on this topic.
Not all cars are the same, but the general rule of thumb is that automatic transmission fluid is checked with the transmission warm and the vehicle on a level surface. Find the transmission's dipstick and remove it, checking that the oil level is up to the "warm" full mark. If it's low, buy the correct fluid for your transmission and top it up with a funnel. Do not overfill and do not spill it anywhere in the engine bay. Remember to have your car checked for transmission leaks if you are concerned.
Low transmission fluid can irreparably damage your automatic transmission. Check the oil level periodically, especially if your car is leaking suspicious fluids. Heat is also a killer and a transmission can overheat if its fluid is low or if it is used beyond its capability, the latter being, for example, towing more than it is rated for. The incorrect transmission fluid can also be problematic. Lastly, don't ignore small transmission problems and have them attended to before they become big ones.
A modern automatic transmission is complicated and has a lot of moving parts. It may have anything from five to ten ratios and its hydraulic system is controlled by intricate electronics. Diagnosing, servicing, taking apart, and repairing them, therefore, requires specialists, the right equipment, and lots of time. Many car transmission problems can be avoided through timeous preventative maintenance though.