Tank
With World War II the multi-turreted tanks proved impracticable, and as the single turret on a low hull design became standard, crews became standardized around a crew of four or five. In those tanks with a fifth crew member, usually three were located in the turret (as described above) while the fifth was most often seated in the hull next to the driver, and operated the hull machine gun in addition to acting as a co-driver or radio operator. Well-designed crew stations, giving proper considerations to comfort and ergonomics, are an important factor in the combat effectiveness of a tank, as it limits fatigue and speeds up individual actions.
Holt Manufacturing Company
As well as having a better power-to-weight ratio, a gasoline-engined tractor required fewer men to operate it. Competing steam engines typically required a crew of seven, including one highly skilled and well-paid, licensed "farm engineer". He was responsible for rising in the middle of the night to start the boiler fire to get up steam by first light. Steam traction boilers had a huge need for water and fuel, either coal or wood. Much of the seven-man crew's time was spent keeping it fueled.