Please settle an argument. If you take your foot off the throttle and put your gear lever into neutral whilst driving down hills, is your car still using any petrol?
Depending on the age and model,If you leave the car in gear with your foot off the throttle it may actually use less fuel than in neutral. Some modern cars will shut off the fuel injectors when there is no throttle applied, If the car is in neutral then fuel is needed to maintain the car at Idle speed.
electronic fuel injection now has the position that when the cars momentum is turning the engine with the throtle not being pressed there will be no fuel used. as soon as the engine revs drop to a point where it needs fuel to tick over , or the throttle is pressed,it will start using fuel again.
leave it in gear and keep your foot off the peddle going down hills and it doesn't burn any expensive go-go juice.
if it's an older car then this may not be true. depends on engine management system.