In an attempt to save gasoline, I’ve noticed that I can get a few extra MPG in the hilly area where I live by shifting my Automatic to Neutral on the downhills. When I shift back to drive, I always ensure that the engine RPM’s come up to match the wheel RPM’s before I give any gas(my car has a Tach). I drive a 2006 Chevy Cobalt. Am I risking harm to my 7-year-old transmission by doing this?
I’m not sure it matters, but how do you know you ‘wheel rpm’, in order to match the tach?
I doubt you could be harming the tranny, compared to some of the other stuff people do that does beat it up.
How exactly did you “notice” this?
How have you “noticed” you get extra MPG? In general, you get worse MPG shifting into neutral because in neutral fuel is needed to keep the engine turning, whereas when you’re in gear the engine is kept rotating my the momentum of the car rolling down the hill and so the engine is using NO fuel and you’re getting infinity miles per gallon. The fuel economy readout does different things in different cars both coasting in neutral and no-fuel coasting in gear, but the above is true in virtually all cars made in the last couple of decades.
I used the fuel mileage readout on the dash. I reset it at the exact same point, and drove the exact same 5 mile route twice. I got 2 MPG better with shifting. I guess with just 2 data points it wasn’t really scientific. As long as I won’t screw up the tranny, I will try a few more runs and collect more data.
And crucible I watch the tach after I shift into drive. When it comes up and stabilizes, I apply gas.
What make and model of car? Different manufacturers do things differently.
Your 2 mpg claim, I am assuming, is only a computer read-out. That is, you haven’t really driven an extra two miles on a gallon of gas, correct? This could be a result of the computer not capturing the fuel use when the car is in neutral. Different computers do things differently.
Is your AC on during this? I ask because my 1999 Chrysler 300M does not shut the fuel down on when decelerating if the AC is on. I can “trick” my Chrysler’s computer by shutting off the AC when decelerating, which will change the computer read-out, but such practices don’t affect the real (fill-up to fill-up MPG) as much as the computer would indicate.
I doubt your actions are giving you much fuel savings. But, that wasn’t your question. Your question was are you hurting your transmission. I doubt you are hurting your transmission unless you are turning your engine off when you shift into neutral. If you turn your engine off, when you restart it, it (the computer) probably assumes you are stationary. Shifting your transmission into drive when the computer thinks the car is stationary can cause problems. But, if you leave your engine running (using gasoline), you shouldn’t be harming anything.
With automatic transmissions, if it feels like you are hurting it (like shifting into drive while you are moving backwards), you probably are.
excavating (for a mind)
The trouble with that is that generally the “AVG MPG” reading isn’t simply a miles driven divided by fuel used measurment. It’s basically a running average of all the instantaneous MPG readings, but only when you’re running at steady speed. It usually throws out the neutral MPG readings (usually zero MPG since you’re stopped) as well as the coasting in gear ones (infinity MPG’s). Different cars have different ways of compensating for both of those situations in order to get an overall average that roughly agrees with what you’d calculate at the pump, but trying to figure out this particular question using the AVG MPG isn’t going to be very accurate.
That said, it is possible with your particular route that the slopes and distances are juuuuuust right so that coasting in neutral allows you to retain more speed at the bottom of the hill which makes up for the small amount of extra fuel you burned on the way down, but that would be unusual. My bet is still on the computer MPG readout being wrong. BTW, it’s also illegal to coast downhill in neutral in most states, although it’s not nearly as dangerous as it was on older cars and I’m not really sure how they’d catch you doing it.
I doubt this. Do you have a cite for this?
Mine can be switched from metric speedo, odo and average and instant fuel usage to “English,” not U.S., because this is Canada. So I thought the readouts in “English” units must be miles per imperial gallon. But figuring it out the old-fashioned way, I found it was neither imperial nor U.S. It’s its very own (I feel so special). So I renamed it GM CFU — General Motors completely fucked up.
Only my own records. Do you want to come and audit them? I keep record for every gas fillup and every mile driven.
Higher octane gas does not affect gas mileage. At lease within margin or error per fillup. On my BMW (V12), I do notice more power, but not better mileage, on higher octane (92 US octane), but on every other vehicle I’ve owned, not even that.
Driving with AC decreases mileage (for me) about 10-15%. I live in Texas, so I only drive without AC for about 3 or 4 months out of the year. Yes, my records show the difference.
Driving at 60 instead of 70 gives is about 10%, maybe 15.
The difference between ethanol added gas an no ethanol gas is below the noise. That is, I can’t see the difference. It may be there, but a tankful of non-ethanol gas doesn’t give me a significantly better mileage than one with ethanol.
My records are my cite. Do you think they are significant enough to publish? If so, who would be interested?
Why do you have any doubt?
excavating (for a mind)
I want to say something about fuel injection systems cutting the fuel flow. I have two vehicles that do this when I take my foot off the gas in gear. However, the Yaris does it much faster that the Ford Ranger does. The Ranger is so slow to shut down the fuel flow that I usually can increase the mileage reading more by coasting in neutral than by keeping it in gear. Not so with the Toyota. This leads me to believe there may be large differences in the timing of the shutoff among other vehicles as well.
I use ScanGauges in the vehicles since they didn’t come stock with mileage gauges.
Maybe the differences I’ve seen people argue about in this thread and others are because of this.
1984 K5 Blazer, 4X4 with a built 350 + auto tranny Has noticeable mileage difference & performance.
Has a regular carburetor, no fancy electronics & the calculation is on many fill ups using the odometer. Many trips on the same 161 mile route, same gas stations, etc.
1989 Chev pickup, 1996 Subaru Impreza, 2004 Mercury Marquis is not as large but we do not have a lot of miles on it as we just got it recently.
All 3 motorcycles, none with fuel injection, have noticeable power & mileage.
One data point from me. Friends with newer cars report same but with smaller values.
IIRC, ethanol fuel has less BTU’s per unit than straight fuel. In controlled test, it does not give the same power - mileage - etc. as non-ethanol fuel.
YMMV 
I don’t doubt your records but you stated as fact that your Chrysler did not cut the fuel on decell with the AC on.
First off this makes no sense from an engineering or emissions standpoint. Secondly if this were true this would be the only vehicle I have heard of in the last 25-30 years that didn’t have fuel cut on decel.
As far as the decreased fuel mileage goes, you do realize that the AC compressor drags several horsepower off the engine (typically quoted as 5-10) and that is probably the reason for the decreased fuel mileage.
Most automatic transmissions are lubricated by a pump driven off the input shaft from the engine. If you shift into neutral and the engine would completely stop, parts of the transmission would spinning without lubrication. Also consider that if the engine is at idle, but the transmission is spinning at 60 mph, there might, just might, not be the proper amount of lubrication.
Check what your owner’s manual says about towing. If it says that the drive wheels must be off the ground, then coasting in neutral could be a bad thing for the transmission.
BTW: Coasting in neutral with a manual transmission, or holding the clutch down, is OK.
One last point: I have a ScanGauge trip computer on my 2007 Kia with an automatic transmission. If I let off the gas at speed, I get a 9999 MPG reading and 0 gallons per hour reading. This means that fuel flow is stopped to the engine. If I put the car in neutral, I’m still using about 0.05 gallons per hour. Of course you could make a point about engine braking slowing down the car faster.
I too have noticed what the OP is talking about. I tried this on the way home from work several times. I stopped doing it because I realised 2 things:
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It would be dangerous if I had to accelerate away from an obstacle next to me or on top of me e.g. a falling tree branch
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It can’t be more than 10-20% more efficient than just leaving it in drive, and with me only rolling downhill for maybe a minute or two over the entire journey, I’d be using 10-20% less fuel for 1 to 2 minutes. Where I live, 1 minute of fuel works out to be about 20c.
So for the slightly increased risk to my safety, I was saving about 2 to 5cents per journey.
I wouldn’t even pick 20c off the ground. I hope this puts it into perspective!
Are you assuming that until you see a change in the instantaneous mileage, the fuel has not been cut off? The computer makes thousands of fuel mixture calculations per second. I cant imagine why there would be any noticeable delay, or how you would know about it if there were.
No. I’m saying the Toyota cuts the fuel immediately after I take my foot off the gas, but the Ford delays the cutoff. I know this by when the gallons per hour gauge goes to zero and when the mileage gauge goes to 9999. Until the Ford cuts the fuel, coasting in neutral gives a better mpg reading than leaving it in gear, and by the time the fuel is shut off, the truck’s almost stopped anyway.
I am not sure, but perhaps this is just a misunderstanding.
When I say “cut fuel”, I mean a total end of fuel delivery to the engine; using cut to mean eliminate. Perhaps when you say “cut fuel”, you mean the rate at which fuel is delivered to the engine is reduced; using cut to mean reduce, as in a tax cut.
In the sense of a reduction in the amount of fuel delivered to the engine, you are correct, when I decelerate in the Chrysler, the fuel delivery to the engine is reduced. With the AC off, it is a rapid, but not instantaneous, reduction and will hit 99 mpg (1 l/100 km), the highest fuel economy the computer will display, within 10-20 seconds of my taking my foot from the gas. I assume this means the engine is no longer receiving any fuel, but being driven by the torque converter.
With the AC on, however, the fuel economy will rarely go to 99 mpg and I’ve never been able to get it to display 1 l/100 km, with the AC on. In the 10-20 seconds where AC off will go to 99 mpg, with AC on it will read 50-65 mpg. Typically, before it reaches 99, I am below 40 mph. I take this to mean that the computer is “cutting fuel”, just not cutting it completely.
From an engineering standpoint, it greatly reduces the amount of engine braking that occurs when you take your foot off the gas, allowing the operator to “coast” for longer stretches, which allows for a smoother ride and can improve fuel economy. From an emissions standpoint, the engine is operating under the highest vacuum, where NOx and unburnt hydrocarbon release is at the lowest, so I can believe not completely turning off the fuel on deceleration can result in lower emissions than you would with a complete cut-off (assuming you were to coast to a stop instead of applying the brake as soon as you take your foot off the gas).
I want to thank you for questioning me on this, since it made me do some research of the engine management system on the Chrysler 3.5 V6 used on my car. The message boards I found had quite a few gearheads who, while not engineers, were quite knowledgeable on fuel delivery programs and how the engine management system for these engines are different than some others, like california jobcase mentions, that produce a total fuel shutdown on deceleration.
You can learn something new everyday, if you look.
No misunderstand about the term fuel cut, when I say fuel cut, I mean the total lack of fuel injection into the engine. I think there is a misunderstanding on your part about how modern fuel injection, computer controls and your trip computer operate.
OK, as far as engine braking goes, your car has a mechanical throttle which does close 100% when you take your foot off the gas. However it has an Idle Air Control Valve which on decell will open to somewhere between 30%-65% for a couple of reason. One of them is to limit the amount of manifold vacuum on deceleration. This is done for a couple of reasons, the main one is to limit oil being pulled up past the rings. By limiting the manifold vacuum, oil consumption goes way down. If you opened the IACV and didn’t have the fuel turned off, the engine speed would increase, not decrease. This is easily demonstrated on your engine with either a scan tool OR just hook up a vacuum gauge. You will not see the 24-25" of vacuum from the days of yore. 17-19" is more like it.
Next from an emission point of view there are two huge reasons to turn off the fuel. The first is in order for the catalytic converter to convert 2HC -> H[sub]2[/sub]O and 2CO[sub]2[/sub] you need 5 oxygen atoms. Where are you going to get them? Simple from turning off the injection every time you lift your foot off the gas, this floods the cat with oxygen which gets stored in the monolith to be used when needed.
Secondly one of the huge problems with cars that had carbs and Bosch K-jet tronic injection was an over abundance of hydrocarbons on closed or trailing throttle. This was because these systems could NOT turn off the fuel on closed throttle.
I think where your misunderstanding comes from a couple of things. First off your engine works one hell of a lot faster than your dash board updates. At 6,000 RPM 1 revolution takes .001 second. Your trip computer is buffered and maybe updates once every second or two. It is entirely possible for you to close the throttle, have all the injectors shut off, open the throttle again again (quickly),have the injectors come back on and notice NO difference in the instant gas mileage due to the buffering and slow update rate of the trip computer. Think about it. Let’s say you are at 6,000 RPM You shut the throttle for .5 seconds and open it back up. The injection process will have stopped for 500 revolutions, but since the buffering and slow update rate on your dash display, you will never see it.
Also with the AC on the ECM will probably modify the program point where the the fuel is turned back on. For example instead of turning on the fuel at 1,235 RPM with the AC off, with it on, the program might turn it back on at 1457RPM (Numbers anally derived,) Also the drag from the AC compressor will bring the engine speed down faster. Both of these will cause the trip computer to make it look like the fuel is not being cut off.
I assure it the fuel is in fact being cut off on your 300
Lastly you said you live in Texas. Texas if famous for many things. Mountains aren’t one of them. Drive this sucker to Colorado and let me know how you do. 
a slowly-turning pump can easily supply enough lubrication to the internal workings of the transmission, but may not be able to supply enough pressure to actually operate things.