Put your gear in neutral when going down-hill in your car. See how far you can go without changing back into gear (safety first though!).
Ellery from Porirua says, “In response to H.C’s tip about using neutral downhill: if the hill is steep enough (e.g. the Ngauranga Gorge), it’s more economical to stay in drive with your foot off the accelerator and let your motion turn the engine without using any fuel at all. If you’re in neutral going down there your engine is idling and burning fuel. I confirmed this with the consumption gauge in my car.”