Why Shouldn't You Coast in Neutral Gear?

[JustinRose]

Here are the reasons why coasting in neutral gear is not recommended for vehicles: When coasting in neutral, the engine stops running, causing the air compressor to cease operation. Since vehicles require frequent braking, the compressed air in the air reservoir continuously depletes, potentially leading to brake failure. It can easily damage the transmission. Most transmissions use rolling bearings extensively. While coasting in neutral, the transmission's secondary shaft rotates, but the constant-mesh gears remain stationary. As a result, these gears cannot agitate the gear oil inside the transmission, causing bearings (especially those positioned higher and relying on splash lubrication) to receive inadequate lubrication. This leads to premature wear of bearings and gears. Braking distance and time increase. Some drivers, aiming to save fuel, depress the clutch and let the engine stall while coasting. When they suddenly release the clutch pedal after descending a slope to restart the engine, power is transmitted in reverse: drive wheels → axle shafts → differential → final drive → propeller shaft → transmission secondary shaft → synchronizer → clutch. The sudden engagement of the clutch with the stationary flywheel subjects the clutch to immense shock loads, potentially damaging the clutch friction plates. According to China's "Road Traffic Safety Law," coasting in neutral downhill or after stalling is illegal. Descending slopes at high speeds without gear resistance increases brake wear and may cause skidding. This regulation was specifically implemented to reduce accidents. Modern electronic fuel injection systems primarily rely on signals from the throttle position sensor and crankshaft speed sensor, with the engine control unit (ECU) determining fuel injection volume. In most vehicles, the ECU commands the fuel pump to stop supplying fuel when coasting in gear. However, when coasting in neutral, the ECU interprets signals from these sensors as indicating an idle state and continues injecting fuel into the cylinders, resulting in wasted fuel. Important Notes: Coasting in neutral doesn't necessarily save fuel because modern vehicles use electronic fuel injection, where the ECU controls fuel injection volume based on the throttle position sensor. When the accelerator is pressed hard, the throttle opens wider, prompting the ECU to increase fuel injection. When the accelerator is released, the throttle returns to the idle position, and the ECU stops fuel injection. If the gear is shifted to neutral, the engine continues injecting fuel to prevent stalling, resulting in idle fuel consumption.