Coasting improve gas mileage?
Here let me start by saying I'm not asking if it is smart or even legal to operate a car in neutral... but I have had an argument along this line.
Putting a car in neutral when going down a hill will increase the mpg more than letting the engine operate as a "brake", i.e., be driven at by the wheels - throttle "closed".
The argument I have heard (besides coasting being illegal and unsafe) is that the engine in neutral is burning fuel to maintain idle at about 750 rpm or so while the engaged engine has the fuel shut down, so even though the rpm may be 3,000, it is using less fuel. I say none sense, in both cases the throttle is saying minimum fuel and in one the engine is running 4 times as fast as the other (coasting/idle) case. Oh yes, the car costing may also go another 1/4 mile or more past the downgrade before the engine has to be again loaded with moving the car.