On modern cars, that actually does more harm than good, especially on diesels. Letting the car idle, by either dipping the clutch, or slipping it into netural, means the engine has to run off its own resources. Think about it, when the cars moving, its essentially driving itself. The wheels are turning because the car is moving, thus driving the crankshaft, and pistons, and the whole process is kind of powering itself. Most modern cars' ECUs will actually partially cut off the fuel to the enigne if it doesn't need it all, so by coasting, you're forcing the engine to run all the fuel to keep the engine running itself, because there's nothing moving it. ...not to mention you have very little control over the speed of the car when you coast. You have no engine braking, which in an emerency, could mean the few extra metres between her and a cabbage-patch-kid.