Why does an international 5-speed gear shift back to neutral?

[Robert]

An international 5-speed gear shifting back to neutral usually occurs when lifting the throttle at high speeds, and there are only two possible reasons: First, the gear lever is not fully engaged (this rarely happens and can be felt by the driver). Second, the teeth of the fifth gear are worn out, causing excessive clearance between the gears and resulting in insufficient meshing, which leads to gear disengagement. Note: This applies to cars that use a manual shift mechanical transmission (also known as manual transmission, MT) to regulate speed. That is, the gearshift lever must be manually moved to change the gear meshing position inside the transmission, altering the gear ratio to achieve speed adjustment. The gearshift lever can only be moved when the clutch is depressed. Functions of a manual transmission: To change the gear ratio, expanding the range of torque and speed variations of the driving wheels to adapt to frequently changing driving conditions. Under the premise that the engine's rotation direction remains unchanged, the reverse gear is used to enable the car to move backward. With the engine running, the neutral gear interrupts power transmission, which is beneficial for engine starting, warming up, idling, and facilitates gear shifting, coasting, or temporary parking. The transmission can also output the engine's power to drive other mechanisms, such as the winch of certain vehicles or the oil pump of dump trucks.