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 advanced technology 737-300, -400, and -500. The engines are larger and mounted forward of the wing struts instead of tucked directly under the wing. For passenger comfort, Boeing gave all three new generation versions the same interior as the 757, with contoured sidewall panels providing additional head and elbow room and a wider aisle compared with earlier airplanes. The 737-300 is about 9 feet longer than the -200 and seats 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.