Why shouldn't you shift to N while the car is rolling?

When using an automatic car, many drivers often have the habit of shifting from D to N to run by inertia before stopping at a red light; however, this action will reduce the life of the gearbox in the long run.

Shifting from D to N while the car is rolling can damage the transmission.

Cars equipped with automatic transmission (AT) are becoming more and more popular in Vietnam. Most new car models on the market have automatic versions. Meanwhile, most drivers in Vietnam when studying and taking driving tests usually start with manual transmission (MT).

Therefore, when switching from a manual car to an automatic car, many people habitually shift to N, let the car run on inertia and brake before stopping at red lights. Some people also have the habit of shifting to N at high speed when driving on the highway, letting the car coast to save fuel.

In fact, with automatic cars, shifting between P, N, D or R while the car is rolling is incorrect. According to an experienced driver, automatic cars have a standard idle speed of 700 rpm or more. When the car is running at a higher engine speed than the standard idle speed, in principle, to avoid affecting the gearbox, the driver should not shift between N, P, D, R. This operation should only be performed when the car has stopped completely, except for automatic cars equipped with manual shift mode M or +/-, the driver can flexibly switch between M and D mode when needing to accelerate or overtake the car in front.

Shifting the gear to N while the car is rolling and letting the car coast before stopping completely, although according to some drivers, will help the car save fuel. However, with traffic conditions in big cities, if you often shift from D to N and vice versa to coast before stopping at red lights, in the long run, it will reduce the life of the gearbox.

“When the driver shifts from D to N or vice versa, the parts inside the gearbox will change their operating state. If this happens frequently, it will cause faster wear and tear, especially on the clutches inside the gearbox, and reduce the lifespan of the gearbox,” said Mr. Dinh Viet Quang - Director of Saigon Ford Service.

In addition, if driving at high speed and shifting to N to coast, some systems on the car will be disabled. This has also been recommended by some manufacturers in the car's manual. In particular, the Mazda3 car's manual, page 46, section "Operating the automatic transmission" warns: "Do not shift to N while the car is running. Shifting to N while the car is running is dangerous because the car will not be able to brake the engine when decelerating and can lead to injury."

In addition, if the engine speed is greater than the idle speed, the driver must not shift from N or P to other gears. The Mazda vehicle owner's manual also states: "Shifting from N or P to other gear positions while the engine speed is greater than the idle speed is very dangerous. The vehicle may move suddenly, leading to an accident."

Thus, the habit of shifting to N to coast when driving an automatic car, in the long run, not only reduces the life of the gearbox but also has the potential to cause danger in traffic situations. Therefore, when driving in an inner-city area with many traffic light systems and having to stop continuously, only in the case of having to wait for a red light for too long, the driver should brake to stop the car completely and then shift to N. Do not shift from D to N while the car is still rolling, to avoid possible damage to the gearbox.

According to TNO