Cars · Lesson 7 of 15Advanced9 min read

Transmission and Drivetrain

An engine makes useful power only within a range of speeds, but a car has to crawl out of a parking space and cruise on the highway. The transmission bridges that gap, and the rest of the drivetrain carries the power to the wheels.

Why cars need gears

When a small gear drives a larger one, the larger gear turns more slowly but with more torque. When a large gear drives a smaller one, the output spins faster with less torque. Power, which is torque times rotational speed, stays the same apart from a little friction. Gears don’t create power; they trade speed for twisting force.

Low gears multiply torque to get the car moving. High gears let the engine turn slowly at highway speed. The differential adds one more fixed reduction, the final drive. Passenger cars typically use final drive ratios of about 3:1 to 5:1.

gear ratio = driven gear teeth ÷ driving gear teeth
output torque ≈ input torque × ratio output speed = input speed ÷ ratio

Set the gear sizes and engine output to see torque and speed trade places. Then take the car around a turn and watch the differential at work. Starting values are examples, not figures for any particular car.

Clutch and manual gearbox

A manual transmission needs a way to disconnect the engine while you change gears. That’s the clutch: a friction disc pinched between the engine’s flywheel and a spring-loaded pressure plate. Pressing the clutch pedal releases the pressure plate so the disc can slip free. Inside the gearbox, pairs of gears with different sizes give each ratio.

Automatic transmissions

An automatic connects to the engine through a torque converter, a fluid coupling with three main parts. The impeller, turned by the engine, flings transmission fluid outward. The fluid strikes the fins of the turbine, which drives the transmission. Between them, a stator on a one-way clutch redirects the returning fluid; at low speed, when the impeller is turning much faster than the turbine, that adds to the torque reaching the turbine. When you’re stopped at a light, the turbine stands still and the converter simply slips. Once the speeds nearly match, many converters engage a lock-up clutch that ties the engine directly to the transmission, cutting losses at steady speeds.

The gears inside are usually planetary gear sets: a sun gear in the middle, planet gears on a carrier around it, and a ring gear with teeth on the inside. One member drives, one is held still, and the third carries the power out; holding different members gives different ratios, and locking two together gives direct drive. Clutches and bands do the holding, and a transmission control module chooses shifts using shift solenoids, reading sensors such as wheel speed, throttle position and fluid temperature.

Continuously variable transmissions (CVTs)

A belt-type CVT has no fixed gears. It uses two pulleys whose halves are cone-shaped and can slide closer or farther apart, with a belt or chain between them. Squeezing a pulley’s halves together pushes the belt to a larger radius; spreading them lets it drop lower. One pulley sets the ratio while the other keeps the belt tight, so the ratio changes smoothly. That lets the engine hold its most efficient or most powerful speed while the car speeds up.

Driveshafts, joints and axles

On rear-wheel-drive vehicles, a hollow steel driveshaft carries power from the transmission back to the rear axle. Universal joints let it work at an angle as the suspension moves, and a slip yoke lets it change length. Front-drive cars use constant-velocity (CV) joints at each end of the front axles, since the front wheels both steer and drive.

The differential

In a turn, the outside wheel travels a bigger circle than the inside wheel, so it has to turn faster. If both wheels were locked to one axle, one tire would have to skid. The differential solves that. The driveshaft’s pinion gear turns a large ring gear, which spins the differential carrier. Inside the carrier, small spider gears drive two side gears, one for each axle. Going straight, the spider gears don’t spin on their shaft and both wheels turn together. In a turn, they turn, letting one side speed up while the other slows by the same amount: the carrier’s speed is always the average of the two wheels.

The catch: an ordinary (open) differential sends the most torque to the wheel with the least traction, so one wheel on ice can spin while the other does nothing. A limited-slip differential sends more torque to the wheel with better grip.

Front, rear, all and four-wheel drive

Front-wheel-drive cars put the engine and a transaxle, a transmission and differential in one unit, over the front wheels. Rear-wheel drive sends power back through a driveshaft to a rear differential. All-wheel drive powers both axles all the time, with a center differential or similar device so the front and rear can turn at different speeds. Traditional part-time four-wheel drive uses a transfer case to lock the front axle to the rear, often with a low range for extra torque; because it has no center differential, it’s meant for loose or slippery surfaces, and running it engaged on dry pavement can damage the drivetrain. Your owner’s manual explains your system.

All- and four-wheel drive are now common: 59 percent of new light-duty vehicles built in 2022 had one or the other. They improve traction, but the extra parts add weight and friction that can lower fuel economy.

Seen from above, front of the car at the top. Sources: U.S. Navy Construction Mechanic Basic, Chapter 11; Wikipedia (secondary) on transaxles and four-wheel drive.

Parts and what they do
Part	What it does	Signs of trouble
Clutch	Connects and disconnects the engine in a manual car.	Slipping: the engine speeds up but the car doesn’t.
Manual gearbox	Pairs of gears give each ratio.	Hard shifting or noises; a technician diagnoses it.
Torque converter	Fluid coupling that multiplies torque at low speed.	A heavy knock can mean a loose converter.
Planetary gear sets	Give an automatic its ratios.	Slipping or harsh shifts; a technician diagnoses it.
Transmission fluid	Lubricates, cools and carries hydraulic pressure.	Red, oily drips.
Driveshaft and U-joints	Carry power to the rear axle at changing angles.	Clunks, vibration, squeaks louder in reverse.
CV joints and axles	Drive the front wheels while they steer.	A torn rubber boot lets grease out and dirt in.
Differential	Lets the wheels turn at different speeds and gives the final reduction.	A hum or whine that stays the same on different pavement.
Transfer case	Splits power between front and rear axles in 4WD.	A technician diagnoses it.

StopWhen to call a professional

Transmission repair takes special tools, the right fluid and the service procedures for your exact transmission; even how to check the fluid differs from car to car, so follow your owner’s manual. Have a technician look at slipping, harsh shifts or red drips. A clunk when you switch from accelerating to slowing down can mean worn slip-yoke splines or other driveline wear.

Check yourself

Tap an answer to check it, or open “Show the answer.”

A 12-tooth gear drives a 36-tooth gear. What happens to the output?
Show the answer
A third the speed, about three times the torque. The ratio is 36 ÷ 12 = 3:1. The output turns one-third as fast with about three times the torque.
Why does a car need a differential?
Show the answer
So the wheels can turn at different speeds in a turn. The outside wheel travels farther in a turn; the differential lets it turn faster than the inside wheel.
In a turn, the inside wheel turns at 90 percent and the outside wheel at 110 percent of the carrier speed. What is the carrier’s speed?
Show the answer
100 percent. The carrier turns at the average of the two wheels: (90 + 110) ÷ 2 = 100 percent.
Which part of a torque converter multiplies torque at low speed?
Show the answer
The stator. The stator redirects fluid returning from the turbine, which adds to the torque at the turbine.
Why shouldn’t part-time four-wheel drive stay engaged on dry pavement?
Show the answer
With no center differential, the drivetrain binds in turns and can be damaged. Front and rear axles travel different distances in a turn. Without a center differential, the tires must skip or the parts strain.

Go deeper

MIT 2.972: How a Torque Converter Works An MIT course page on how a torque converter works.
Northwestern University Mechatronics Wiki: Continuously Variable Transmission A Northwestern engineering wiki on CVTs, with diagrams.
U.S. Navy, Construction Mechanic Basic (NAVEDTRA 14264A), Chapter 11: Drive Lines, Differentials, Drive Axles, and Power Train Accessories The Navy’s chapter on drive lines, differentials and axles.
OpenStax University Physics Volume 1, 10.8 Work and Power for Rotational Motion Rotational power, the physics behind gearing.

Sources

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A free course from Teddy Rook Book. It explains how cars work in general and follows U.S. sources; your owner’s manual and your vehicle’s service information are the final word for your car, along with state and local rules.