Why Shouldn't You Coast in Neutral Downhill?

[McKira]

When coasting in neutral, the car loses the engine's braking effect. During downhill driving, due to gravity, the vehicle's speed will increase continuously. Without the engine's braking force, the braking distance will be significantly extended, meaning the effectiveness of the brakes will be greatly reduced, putting the vehicle in an uncontrolled state and creating a major safety hazard. Below are the relevant explanations: 1. When the vehicle is coasting in neutral, the engine is dragged along by the vehicle's inertia, causing the drive wheels to lose restraint. This can lead to the vehicle accelerating uncontrollably downhill. Relying solely on the braking system to decelerate will rapidly increase the temperature of the brake pads, significantly reducing braking efficiency and potentially causing brake failure, which is extremely dangerous. Conversely, shifting into a lower gear allows the engine to assist in braking. When the engine is running, it generates substantial resistance, naturally reducing the vehicle's speed. This helps balance the vehicle's resistance and inertia, ensuring more stable driving. 2. Transmissions are divided into manual and automatic types. For manual transmissions, when coasting in neutral, the engine idles while the coasting drive wheels spin the transmission gears at high speed, creating a speed differential. Prolonged operation under these conditions can lead to inadequate lubrication and damage the transmission. For automatic transmissions, coasting in neutral similarly generates excessive heat inside the transmission. Automatic transmissions rely on the oil pump to circulate fluid and dissipate heat through the radiator. If heat cannot be dissipated promptly, the automatic transmission may overheat and fail. 3. Modern cars predominantly use electronic fuel injection engines, where fuel delivery is controlled by the throttle and ECU. When the accelerator pedal is released, the ECU reduces the fuel injector's pulse width or cuts off fuel injection at higher speeds. However, when coasting in neutral, the engine remains at idle and continues to inject a certain amount of fuel to prevent stalling. Therefore, coasting in neutral does not actually save fuel.