The powertrain is one of a car's most complicated parts. The term "powertrain" refers to the system of bearings, shafts, and gears that transmit the engine's power to the axle. Plastics help reduce the number of parts needed to assemble these complex components. Plastics also help reduce vehicle weight, which helps lower assembly costs while increasing fuel efficiency.
The two sets of engine powertrain gears that transfer power to the drive wheels are the transmission and the differential, discussed in more detail on the pages linked below.