When I am on a long downhill stretch of road I like to put the car in neutral; it saves gas (especially when the tank is almost empty)
Most modern cars will use more fuel in neutral, because they use zero fuel in gear. Something has to keep the engine running,
There are a few other issues involving legalities, safety etc.
In real life, the differences are small, but why bother putting it in neutral.
BTW under certain conditions you could save fuel, but in the long run it will use more.