system
Given the following post, respond as if you're commenting on Reddit:
human
[eli5] My friend put the car to neutral when coming to a stop light. He says it saves gas and it stops smoother. I agree on the smoother part, but does it actually save gas? He also put it to neutral when waiting for the lights to turn green for the same reason. Is it true?
gpt
On most modern cars, when decelerating in gear, the ECU that controls the engine will actually cut fuel completely.  meaning you'll actually use **less** fuel by decelerating in gear than you would by idling in neutral while you roll to a stop.

Also, as others said, you'll also need to use more brakes without any help from the engine drag, meaning additional brake pad wear.

basically by decelerating in gear, your engine turns into a big ol air pump, using the momentum of the car to compress air and then release it.  You dont need (or want) any gas in the chamber for this.