the transmission with the rear axle gearing. Four-wheel-drive
vehicles have differentials on both their front and
rear axles.
The differential allows the drive wheels to rotate at different
speeds when negotiating a turn or curve in the road
and redirects the engine torque from the drive shaft to
the rear drive axle shafts. The drive shaft turns in a motion
perpendicular to the rotation of the drive wheels.
The final drive gears redirect the torque so that the drive
axle shafts turn in a motion parallel to the rotation of the
drive wheels.
The final drive gears in the drive axle assembly are also
sized to provide a gear reduction, or a torque multiplication.
Axles with a low (numerically high) gear ratio allow
for fast acceleration and good pulling power. Axles with
high gear ratios allow the engine to run slower at any
given speed, resulting in better fuel conservation.