The habit of turning the ignition off to save fuel downhill is a bad idea because it can lead to a crash.
Steering Lock – you will be unable to turn until you restart the engine.
Braking – your brakes rely on the running engine and are considerably less effective without it.
Safety Systems – safety systems like ABS would also be disabled.
Putting the vehicle in neutral
Coasting wastes a lot of fuel. In neutral, the engine has to run at idle speed in order to keep itself going.
In low gear, the engine is kept turning by the car’s motion, which helps to slow the car.
This actually uses zero fuel.
The effectiveness of your brakes is reduced when in neutral, as you don’t get much vacuum assist at idle speeds.
In general, the heavier the vehicle, the greater the role of the engine is.