Don't do it.Whenever you are in neutral you are not in full control. Just keep it in a high gear if you are in undulating country and don't let the revs drop below 2,000rpm. Going down a steepish hill I am more concerned about control and use the gears to hold the car at a speed I want with occasional touch on the brakes to keep it in check. That 7.4L/100 km is great BUT was it just on the car's computer? If so it is only giving you a momentary glimpse of the fuel consumption. When you fill up is when you know the whole truth.