This is one of those topics of discussion that most people get wrong. In older petrol cars that used carbs, coasting or freewheeling downhill would save you fuel, and this method worked quite well for those older cars. Modern petrol cars with Electronic Fuel Injection, do not work the same way. Modern cars with EFI have a Fuel Return Pipe which sends unused fuel back to the tank, so if you just simply leave the car in gear, and keep your foot off the go pedal while on a downhill stretch of road, the engine will not be using any fuel. If you were to use the old technique of coasting in neutral on a downhill stretch in a modern EFI car, when the revs drop below approx 1000rpm, the tickover valve kicks in, and you are burning fuel to keep the engine running. So the best way to save fuel in modern petrol cars is to effectively coast in gear, and when slowing down, use engine braking as much as possible, by simply going down through the gears, to keep the revs above tickover, with the fuel supply cut off, then when you have almost stopped, you simply have to let the engine reach idle speed. Most diesel cars have a Fuel Cut Off Valve, that works in a similar manner by simply cutting off the fuel supply when you take your foot off the pedal, so the same method of coasting works for both EFI & diesel cars.

For more Fuel Economy Tips, please read the other posts.