The drive train (drivetrain or powertrain) is a series of parts that
transfer energy from the combustion engine to the wheels. The drive train has three
main parts: the transmission, drive line or drive shaft, and
differential. ln, some four-wheel drive vehicles a transfer case is also
necessary.
The powertrain for a Subaru vehicle. The engine and transmission are seen
on the right. The drive shaft is in the middle and the differential is
on the left at the center of the rear axle.
The rotation of the crankshaft in the internal combustion engine does
not equal the rotation of the wheels. Just like a bicycle, motor
vehicles have gears. The lower gears are used for lower speeds, while
the higher gears are used for higher speeds. Yet, the RPM which is read
on the dashboard tachometer may be relatively constant (or stay within
a range) through all speeds. The transmission, which is where the
actual gears are located, takes an input speed from the crankshaft and
reduces it to the output speed until the vehicle is in “direct” drive.
ln other words, it changes the gear ratios. So, in first gear, you might
have an input-to-output ratio of 4 to 1. This means that as the input
shaft goes around four times, the output shaft goes around one time. At
4,000 RPMs input, you would have 1,000 RPMs output. The second gear may have
a 3 to 1 ratio and the third gear a 2 to 1 ratio. The fourth gear would be
direct drive with a 1 to 1 ratio.
Think of the sprockets on a bicycle. There is a large sprocket on the
front that rotates as the bike is pedaled, and there are also large and
small sprockets on the back wheel with a chain that connects the front
sprocket to the rear sprockets. As the gears are changed, so is the
mechanical advantage for the person that is pedaling.
ln direct drive, the input speed, and the output speed are the same;
there is no gear reduction. Modern vehicles have overdrive, which is a
ratio of 0.75 to 1. This means that the input speed is less than the
output speed, which results in better fuel economy on modern vehicles.
These numbers and ratios are used to present the general idea. Every
vehicle has its own input-to-output ratio.
The drive line (or drive shaft) is a round tube that transfers power
from the transmission to a differential. lt is situated longitudinally
underneath the car. A front-wheel drive vehicle won’t have a drive
shaft. Some four-wheel drive vehicles have more than one drive shaft.
A differential (aka rear differential in rear-wheel drive vehicles,
rear end, or part of the rear axle) has hypoid gears. These are
specialized gears that transmit the rotation of the drive shaft 90
degrees in order to turn the axles and correlating wheels and tires.
There are spider gears inside the center section of the differential
that allow the wheels to travel different distances while turning.
Consider a car that is turning right. The driver’s side rear wheel
travels a farther distance than the passenger side rear wheel. Think of
using a protractor to draw a circle. One leg of the protractor simply
rotates on one central spot, while the other leg draws the circle. While
the rear wheels of a vehicle don’t turn as sharply as a protractor, the
principle is the same.
A differential for a German vehicle with cutout to see the inside.
 
An illustration of a differential that shows how the gears can be used
to change the direction of the rotation. The drive shaft is in the
lower right corner of the image. The top right and lower left of the
image show the rear axle which extends to the rear wheels.
The Difference Between a Manual and Automatic Transmission:
A manual transmission (which is also called a standard transmission)
uses conventional gears. All of these gears are turning at all times
when the engine is running and the clutch is released. When a gear is
selected with the gear shift, the gear is attached to the output
shaft. Then to change gears, the clutch is depressed and the next gear
is selected which attaches that gear to the output shaft. The gears are
manually selected.
Illustration of a manual transmission.
An automatic transmission uses “sun and planet” gears. A “sun and
planet” gear set has a central “sun” gear with “planetary” gears
(typically three or four) located around the “sun” gear. These gears
can rotate, turn or hold to create different speeds and directions. To
select the gears there are multiple clutch packs and/or bands that are
hydraulically applied by a valve body to control gear selection.
Illustration of sun and planet gears in an automatic transmission. The
yellow gear in the illustration is the “sun” gear and the four violet
gears surrounding the sun gear are the “planet” gears.
Types of Automatic Transmission Fluid:
There are 30 or more different types of transmission fluid for today’s
modern vehicles. All the fluids look the same. Refer to the owner’s
manual to select the correct fluid.
Aftermarket suppliers make fluids that are compatible with multiple
different vehicles. When choosing aftermarket fluids, be sure that the
specified fluid in the owner’s manual is listed on the aftermarket
bottle.
Lubricants Used in Standard Transmission:
The lubricants used in a standard (or manual) transmission on modern
vehicles have multiple types of fluid. Be sure that the fluid installed
in the vehicle meets the manufacturer's specifications.
Lubricants Used in Differentials:
Most differentials use conventional 80-90 gear oil. The 80-90 numbers
indicate the weight and viscosity of the oil. Some vehicles require special
fluids. Be sure the fluid meets the manufacturer’s specifications.
Limited slip differentials may require an additional additive.
The Difference Between Front Wheel Drive, Rear Wheel Drive & Four
Wheel Drive:
The main differences between front-wheel, rear-wheel, and four-wheel
drive are the position of the drive train parts and where the drive
train transmits the energy.
Illustration of a transaxle that has a transmission and a differential 
Front-wheel drive
Includes engine (red), and transaxle.
A front wheel drive vehicle incorporates a differential and a
transmission into one piece, called a transaxle. The transaxle
transmits power from the engine to the front wheels.
Rear-wheel drive
Includes the engine (red), transmission, drive shaft, and rear
differential.
A rear-wheel drive vehicle transmits engine power through a
transmission to the drive shaft, and then to a differential in the rear
of the vehicle.
Four-wheel drive
Includes engine (red), transmission, transfer case, front & rear
drive lines, and front & rear differentials.
Alternate four-wheel drive
Includes engine (red), front transaxle, drive line, and rear differential.
One type of four-wheel drive vehicle transmits power through a
transmission, and then to a transfer case, which distributes the power
through the front and rear drive shafts, then to the front and rear
differentials. Another type of four-wheel drive has a front transaxle and
rear differential.
(The diagrams in this book are not the only configurations. A vehicle can
have a rear engine with all three drive types, or even a mid-mounted
engine.)