coasting in gear uses 0 gas?, & gear oils? - Fuelly Forums

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Old 07-02-2008, 11:19 AM   #1
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coasting in gear uses 0 gas?, & gear oils?

I saw in a top gear episode if you coast in gear to a red light it uses no gasoline while doing so (in a modern engine) if true can someone explain why. also would doing this shorten the life of me std tranny?

Another ?.
would a lighter gear oil for my manual transmition help my mpg. i own a 97 civic dx. if so whats the best?

thanks
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Old 07-02-2008, 11:42 AM   #2
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I don't know about your gear oil question. The answer to your other question is that it's true under certain conditions. It's called Deceleration Fuel Cut Off (DFCO for short). Google for it if you want.

In my 2008 VW, if I have my foot off the gas and it's in gear above 40mph, it does DFCO instantly. If I'm between 35 and 40, it may take a couple seconds before it goes to DFCO. Between 30 and 35, it is inconsistence. Below 30 it does not DFCO under any condition.

However, once it's in DFCO mode, it stays that way all the way down to 1000rpm or ~17mph, whichever comes first (obviously depends on what gear I'm in).

My 2002 GMC takes 6 to 10 seconds at >1500rpm before it will DFCO, and holds it down to 1000rpm.

I am sure of these behaviors because of my fuel rate monitor (as linked in my sig).

DFCO itself won't affect the life of the transmission any more than if you kept your foot on the gas on the way up to that traffic light. If you have to shift to make it happen, well you're aware of how much wear you'll get from that shift -- not enough to worry about. If you are in neutral (due to P&G strategy, most likely) then you probably won't want to rev-match, which may delay DFCO, so you'll get a normal amount of clutch and synchro wear. Altogether I don't worry about it at all.

If you intend to slow or stop, might as well try to DFCO while you do it!
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Old 07-02-2008, 01:06 PM   #3
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what happens is the fuel injector is cut out so the wheels are turning the engine, this will cause you to slow down, but not much, especially if you're in tall gear. if you're going slow you can get quite far by doing this.

just practice keeping you're foot off the gass as much as possible, and keep an eye on the speed... it may feel as if you're suddenly loosing power and slowing down, but if you look at the speed you'll notice you can do this and still do the same speed as you would with your foot on the gas under certain situations... if the speed goes so low and you still have some distance to cover just lightly tap the gas and coast.

don't know about the transmission, mines sealed for life, so i'm not gonna mess with it... use the talles gear possible, that will keep the revvs of the transmission down, in theory that should cause less wear.
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Old 07-02-2008, 04:14 PM   #4
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Quote:
Originally Posted by Mark_alot View Post
I saw in a top gear episode if you coast in gear to a red light it uses no gasoline while doing so (in a modern engine) if true can someone explain why.
As another poster mentioned, you have to be in gear, with your foot off the gas, with enough RPMs (with many cars it's somewhere around 1200 RPMs, but this does vary with the car) to make your engine computer realize that the fuel isn't doing much for you.

The latter point is key, as when your RPMs drop enough, you will lose the fuel cutoff (and go back to using gas). However, in some cases you can downshift (causing even quicker engine braking slowdown) to go back into the fuel cutoff mode for a while. For example, on my CRX I'll often start the coast into a red light in 5th/high gear. But as soon as I feel the slight "bump" (usually happens around 30MPH on my CRX), that accompanies the engine computer using fuel again, I will often shift down to 3rd gear and again get fuel cutoff (because the down shifting raised my RPMs above the fuel cutoff point) for several seconds more (assuming I've still got a ways to go before the red light).

Quote:
Originally Posted by Mark_alot View Post
also would doing this shorten the life of me std tranny?
Probably not, as long as your transmission is in good shape. After all, you are just using your transmission in reverse in this case (i.e. instead of the engine providing power to the wheels, with fuel cutoff the motion of the wheels are turning the engine).

Now, if you really care about your transmission, I would recommend replacing the transmission fluid, with quality transmission fluid, at least every few years. IMHO transmission fluid is an area many people neglect in a car, yet proper tranny lubrication can make a huge difference in tranny wear.

And proper "fresh" tranny fluid can even help FE some, as less friction in the transmission, results in your engine working just a little less hard to turn the wheels!

NOTE: That's one reason why I (personally) now just use synthetic lubricants in my transmissions. After all, synthetic lubes might not make a huge difference in FE, but they do lubricate better than conventional fluids. And that extra lubrication (and therefore slightly less heat/friction in the tranny) has to be helping FE at least a little bit.

Quote:
Originally Posted by Mark_alot View Post
Another ?.
would a lighter gear oil for my manual transmition help my mpg.
Possibly. However, it might also risk additional wear in the transmission.

Personally what I recommend (and what I do myself), is to get a quality SYNTHETIC transmission fluid that is designed for use in your type of transmission. Do NOT use "regular oil" even if/when your owners manual says it's OK, as the additive mix that makes a good motor oil is NOT the same as the additive mix that makes a good transmission lubricant (as the needs of an engine are different than the needs of a transmission).

For example, I use "Redline MTL" (a synthetic transmission lube designed for manual transmissions, such as the one in my CRX, that were originally speced for 10w30 car motor oil) in my 1991 CRX, and it's been extremely happy with it (I can't tell you how smooth she shifts, since I put that stuff in your manual transmission). Yes, the stuff was over $8/quart, but my manual transmission uses less than 3 quarts total (so it didn't exactly break the bank).

Quote:
Originally Posted by Mark_alot View Post
i own a 97 civic dx. if so whats the best?
I had to make a few assumptions, as you didn't give complete info on which car you had. For example, I'm assuming by "std tranny", you were talking about a MANUAL (i.e. stick shift) transmission. Correct? If so, you will want a good (synthetic) manual transmission lube. The trouble is, which lube is best varies a bit with the design of your manual transmission (so we need more info, to proper suggest which manual transmission lube would be optimal for your tranny).

Personally, based upon the research that I did when I was picking out a transmission fluid for my CRX, I would go with either a synthetic offering from "Redline" or "Amsoil" for manual transmissions (as those two companies seem to have a reputation for making good synthetic transmission fluids). FWIW: I actually went with the Redline transmission fluid in my CRX, and I'm very happy with the results (it's unreal how huge of a shifting difference a good transmission fluid can make). However, it was somethiing of a toss-up, as both company's offerings were about the same price, and both were reported (on places such as the "Bob is the Oil Guy" forums) to make quality synthetic lubricants specifically designed for transmission use.

As for automatic transmissions IMHO the choice is easier, as I'm a great fan (based upon both the specs, and personal experience with using it in the family vehicles with auto-transmissions) of how well Amsoil's "Synthetic Universal Transmission Fluid" (i.e. the almost "universal" synthetic ATF that Amsoil makes) works. That fluid was designed to exceed the specs of multiple other types of ATFs, and therefore can be used as a good/safe replacement for many different ATF formulas. At this point, I pretty much just use that Amsoil ATF (in the auto-transmissions of) any home vehicle that Amsoil lists that ATF as being OK for (which is most, but not all, brands of auto-transmissions out there).

FWIW: I did cross-reference your car (again, assuming a manual transmission) against the Amsoil site, and FWIW they suggested using their "AMSOIL Synthetic Synchromesh Transmission Fluid" in 1997 Civic manual transmissions (I suggest using Amsoil's ATF, if you have an auto-transmission). I don't have personal experience with that specific (manual transmission) lubricant, but I have used several of the other Amsoil synthetic lubricants (including their excellent synthetic ATF) with good results. So if you want to try the Amsoil offerings (for your car), feel free to send me a PM (as I went to the trouble of becoming an Amsoil dealer, so I'm in a position to offer gassavers members some discounts vs normal Amsoil prices).
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Old 07-03-2008, 03:54 AM   #5
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so when ever im coasting i put the car in hi gear 5th and thats it.
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Old 07-03-2008, 05:05 AM   #6
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It's not really coasting, you do that in neutral. Your objective is to use downshifting to keep engine speed above the DFCO threshold, which uses no fuel compared to coasting which uses fuel because the engine is idling.

I downshift as the car slows down to keep DFCO engaged for as long as possible.

I know my car uses DFCO down to 860 RPM with no loads that would increase the idle speed. With AC on the RPM rises to 1300, so I have to adjust my downshift points accordingly.

This is not hard on the transmission, when you are downshifting at engine speeds below 1500 RPM. In many cases I will downshift 5-4-3-2, but I don't bother with 1st gear.

The extent of the downshifting is totally dependent on the red light I am trying to time so that I arrive after the light has changed and traffic is moving away from the light. Ideally I end up in 5th gear, but as it becomes apparent I will need a lower gear to accelerate from the green light, then I use lower gears. I want to og from DFCO directly to acceleration in the same gear when I can delay my arrival at the light until I avoid any unnecessary deceleration.

regards
gary
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Old 07-03-2008, 06:39 AM   #7
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the DFCO point will vary from car to car. I've got an 06 Ion that around 1500 RPM's in 5th gear I feel a bump bump, which I think is the fuel injectors cutting back on. In lower gears around that same RPM's I can't feel it. I do not have a scangauge or other monitor so I don't know for sure if what I feel in 5th is it but my educated guess tells me it is. I haven't broken 40 MPG from one tank of gas yet but getting close. This tank right now seems to be going well and should net high 30's as I have been lately anyway, but this time seems like I've gotten farther out of 3/4 of a tank than I usually do.

Depending on what gear lube they use will depend on what you can do with it. The Saturns use ATF in their manual trannies. Yes ATF. That is already pretty thin stuff. Whether or not I decide to actually step it up to a synthetic is to be seen but I doubt it would result in any gains.
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Old 07-03-2008, 06:42 AM   #8
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My 96 Civic DX does fuel-cut in gear down to 1200 rpm, then it fires up the injectors. You can feel a "surge" when that happens.
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Old 07-03-2008, 07:57 AM   #9
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It's perfectly fine to do. Why would it damage/wear the transmission any more than when you talk about it in a different context?
http://en.wikipedia.org/wiki/Engine_braking
http://theautoprophet.blogspot.com/2...e-braking.html
http://en.wikibooks.org/wiki/Transwi...ession_braking

It's an old question and large quantities of discussions can be found about it if you just look it up as "engine braking" rather than "DFCO".
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Old 07-03-2008, 08:11 AM   #10
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What's slowing down the car is that your engine is still compressing the air.

No, you don't have to be in fifth gear. It happens in every gear.

Factory tachometers are not accurate enough (they move too slow) to give you an accurate indication of the fuel cut-off RPM. For instance, someone said it was 1200 in their 96 DX but it's actually 990 RPM according to the Helms manual.
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