Development of the quieter, more fuel-efficient CFM56-3 series of engines with appropriate thrust allowed Boeing to improve its most popular jetliners and produce the Classic 737-300, -400 and -500. The engines are larger and mounted forward of the wing struts instead of tucked directly under the wing. The 737-300 is about 9 feet longer than the -200 it was designed to replace, and can seat up to 21 more passengers. The technological advancements combine to lower operating costs and yield a 30 percent reduction in fuel burn per seat compared with the 737-200.