Questions around my idea for saving fuel when driving- would this work?
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!!!
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Filed under Gas Conservation FAQ by on Oct 21st, 2011. Comment. 
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Comments on Questions around my idea for saving fuel when driving- would this work?
Coasting downhill in N is dangerous, plus you will be riding the brakes and wearing them out a lot faster.
Freewheeling when going down a hill is not a safe practice and for larger vehicles
it would be dangerous…. Engine rpms and compression ratio are there to control
deceleration and the vast majority of the motoring public would be crashing and
going over the side of hills and stuff.. check out the guys on the internet that are
getting unbelievable fuel mileage like a 100 miles to the gallon… they practice all
the things you are suggesting…
that really is a good idea,
i drive a manual and in the same conditions put the car into neutral and DO have a slight fuel saving on just one journey, with the mileage i do per year if i was doing the same route twice a day i could save nearly a full tank plus engine wear and tear.
it would definitely be worth your while talking to an auto electrician to see if he could wire something up. it could work along the same lines as hybrid engines where the electric motor kicks in, even as a re-charge for an electric motor using a dynamo or something similar
Yes it will save fuel but you’re not going far enough. Extreme hypermilers don’t just coast downhill, they turn the engine off. They even sometimes turn it off in traffic and coast. It’s very dangerous but if your objective is to get extreme mileage, sometimes about 100 mpg in a standard car, that’s one of the things you need to do.
ive heard that when ur off the throttle it only uses as little gas as it needs to keep it running and that its worse to put it in N but logically i would think it would save gas.
I can see the theory you are getting at. Even as a little kid I used to wonder why my parents car with automatic transmission had to run so fast at 60-70 miles per hour in hi gear. Sure enough, a few years later the factories added 4th gear and a lockup torque converter that solved the problem. Now, some electric cars are charging their batteries as they coast by using the momentum to run a charger. I think in your theory you just don’t go downhill often enough for coasting in neutral to add enough mileage to make a difference unless you live at the top of a mountain.