Running in neutral

My son has an automatic transmission and says he read that you can put the car in neutral and cost down hills and to a stop sign in order to save gas. Will this hurt the transmission if you are moving when it is put back into gear at a high rate of speed? It’s nice to save the gas but not at the expense of a transmission. Thank you for your help.

It’s unsafe, it’s illegal in many places, and it causes most cars to use more gas.

In addition to what lion9car correctly stated, I will add that your son’s driving style causes a lot of extra wear on the transmission. Each time that you put the transmission into gear, clutches and other parts slam into place together. Thus, every time that he slips the trans into neutral on a downgrade and then puts it back into gear when he hits level ground, he is adding wear and tear to that very expensive transmission.

Even if he was saving gas with this transmission tomfoolery, the meager savings on gas would be far outweighed by the probability of having to overhaul or replace the transmission much sooner than would be necessary if he was operating the transmission as it is supposed to be operated.
But, since he is NOT saving gas with this tactic, this is truly a lose-lose situation.

This is a very bad idea, no matter how you consider it.

There are ‘hyper-milers’ who do that, but they seem to have an intimate relationship with their vehicles and know just what they can and cannot get away with.

I shift into neutral at stop lights and, over enough time that does add-up to some mileage, but if he doesn’t do a significant amount of driving, sticking with what the engineers gave your son is probably best.

Every time you shift in and out of drive, all of the drivetrain components take a jolt. These components are designed to take this impact, but shifting in and out of drive accelerates this wear and could lead to premature failure of one or more of the drivetrain components.

In most modern cars, coasting in gear causes fuel to be cut off. Coasting in neutral means the engine must use gas to keep the engine running.

In summary, your son is wearing out his transmission and wasting gas.