In a car, the engine makes the crankshaft spin. The crankshaft makes the transmission's gears spin, which spin some more parts, which make the wheels spin. But the wheels do not spin as fast as the engine makes the crankshaft spin. That is because the gears are made to slow down the spinning. The gears also give the spinning more force (or to be more scientifically exact, more torque), so the wheels can make the entire car move. most transmissions have a breather vent to let hot air escape.