The core question is, when your foot is OFF the gas and you are coasting down a hill (auto transmission), is fuel still being burned at about the same rate it would be under a level ground condition under moderate throttle- AT THE SAME RPM? I understand there is more load going up a hill or even on level ground than going down but I am trying to understand again if going down a hill, with the RPM’s being the same, am I burning more fuel than I would be at an idle condition. Are the RPM’s only high because they are holding the car back, like a mechanically high RPM- does the computer know not to add fuel during this type of driving condition? The TPS (throttle positioning sensor) should know that no gas input is happening, so again, in this downhill scenario, when the RPM’s are typically staying up around 2, 300 rpm, is the vehicle burning 2, 300 rpm worth of gas (is fuel being pumped in more than at an idle rate) OR is the engine simply causing the revs to be higher, like downshifting with no additional fuel being burned (other than an idle-like rate of flow)? Does the computer know not to add fuel?

Here is why I ask…and my idea for saving fuel. I have noticed that if, e.g., I am coasting down a long mountain road, my RPM’s are about 2, 300. If I move the shifter into Neutral, they drop to about 800- a net savings of 1, 500 rpm. IF in fact I am saving 1, 500 worth of fuel savings, that would be significant. A simple “Green Switch” on the steering wheel could then be used to toggle the transmission into this “Neutral like state.” The system could be designed to automatically put you back in the range as soon as you hit the gas peddle- similar to how cruise control works- as soon as you hit the brake, it shuts off. Hills offer kinetic energy that could be used to save fuel. Logically, 1/2 of our driving time is spent going up hills and 1/2 of our driving time is spent going back down those same hills- assuming a person makes the same daily commute.

THANKS!
THANKS FOR THE ANSWERS- I am not concerned about going through more brakes and although I definitely agree that freewheeling would be dangerous, I am sure a simple solution could be designed to hold the car back on hills that did not involve using gas.

My core question remains, is fuel being added to the engine at 2, 300 rpm’s going down a hill at the rate of 2, 300 rpm’s or at the rate of idle (~800 rpm’s) even thought the tach shows 2, 300.

My idea seems to have promise but I might be wrong. One of the answers made me think to compare EPA estimates on modern AUTO vs. Manuals are they are about the same- in some cases the auto’s actually achieve higher fuel economy. There is no dramatic difference and I’m sure many people do use the technique of coasting down hills when they are driving manuals transmission vehicles in an attempt to save fuel.

THANKS FOR ALL YOUR GREAT AND HELPFUL ANSWERS!!!