So I've learned a new trick a while back to save up on gas while the gas prices were going into the mid&upper $4 range. The trick is to throw the transmission into Neutral whenever I'm in a situation where the throttle isn't needed. Example of situations would be when I would brake from highway speed to exit; when I notice traffic ahead of me and must gradually reduce speed; braking towards a stoplight from full street speed; etc.

The Theory: Whenever the vehicle slows down, the transmission goes through a series of downshifts. Whenever the throttle is released, the transmission runs the engine's RPM, steadily slowing the RPM down. Higher RPMs means more gas is burned (per minute, so to speak). So to prevent this, Whenever I'd be free-rolling or slowing down, the transmission is put into Neutral which drops the RPM down to about 600-700 (as opposed to 1500-2000 at a comfortable cruising speed).

So ordinarily, the engine will stay above 1500rpm during all those downshifts; if you were at 6th gear, that means that the engine stays above 1500 until you come to a halt and then the engine idles at 500rpm. But if you were to throw the transmission into Neutral, the rpm would drop right down to 600-700 and you would just reduce your speed and the engine will stay at a low 600-700rpm range.

Now, if we were to add up all the time we spend slowing down, braking, or cruising* (*not cruise control, but just letting the car roll with foot off the throttle*), that's quite a bit of time; which means, quite a bit of time that the engine will be well above 1500rpm. But lets say for all those times, the tranny was thrown into N and the engine can stay at 650rpm instead. That's less than half the rpm as normal, which means less than half the consumption for all those times that we spend slowing down, braking, and cruising*.

Now, I've been driving like this for a while and the results...28-32mpg on average. Now tell me, is this hazardous to the transmission? If the question of gears engaging during high speed is the concern, think about when we shift into D from P. The gears engage, and we would occasionally feel a jolt as the gears engage. I think that the wear on the transmission is just as much wear as any other case. And plus, our transmissions don't even let us to a proper burnout by revving at 3000rpm on N at start and throwing the shifter into D (the tranny actually waits until the rpm drops before the gears engage) believe me, I've tried it. If not, see for yourself. lol Go out on the street, idle at N, rev the engine, and throw the tranny into D while the rpm is 3000 or higher; the tranny will wait till the engine drops in rpm.

But the point of it is...Is this a secret trick to fuel conservation, or is it a hazard that'll shorten the life of the transmission?

 

Well, this is a good question, however, your assumption that less RPM's = less gas being used is incorrect. You have to go into the mechanics of a vehicle to understand if this is good.

The fact is, aside from the whole safety perspective of coasting in neutral (less stopping power - using more brake pads to stop - more friciton, less rotor and pad life) and not having the transmission aide in braking (what if someone cuts you off in an emergency) plus, the car doesn't handle as well when in neutral vs transmission engaged. You won't save any $ in the longrun.

But lets get into the myth of saving gas. Your car's consumption meter uses an algorithm that is not based on how much fuel is actually consumed, but rather, using calculated values based on airflow past the mass airflow sensor, manifold vacuum and engine rpm. And it's not accurate under these coasting conditions. This is for the simple fact that RPM does not always mean fuel is being consumed! (I know, this might shock most people).

With tests using a scan tool to measure pulse widths (throttle inputs to tell the engine to engage injectors and add gasoline) the pulse widths vary from 5 percent or so at idle to around 80 percent or so at full throttle. The higher the percentage (ratio) of on time to off time, the more fuel used. There's one on pulse for every cylinder firing, so the consumption also varies with engine speed (rpm). All vehicles show a short pulse width at idle, sitting in traffic at a red light at idle or coasting in idle — IN NEUTRAL. The vehicle in drive will use slightly more fuel sitting in drive at a traffic light, because of the drag in the torque converter, and inefficiencies in the drivetrain.

However, interesting most vehicles show a pulse width of ZERO when coasting while in gear (drive). Zero, as in there is no fuel injected at all. Yes, the engine is turning over (RPM's), the pistons are going up and down, the water pump, alternator and a/c compressor are working, so technically you can say the engine is running, sort of. But it's not consuming any fuel. And that goes for automatic or manuals.

These findings were not performed on a mercedes-benz, but I am fairly certain the results will remain the same.

While the trip fuel economy cluster calculator may show you are saving fuel. I doubt you are actually REALLY saving any fuel by putting the vehicle in neutral while coasting. In fact, research shows you are using MORE fuel.

The only way that this practice will save fuel is if the drivetrain power losses require more fuel while coasting than the engine uses at idle. Which is rare for well designed transmissions, let alone mercedes transmissions. (So no, you aren't saving fuel).

Lastly, you DEFINITELY won't be saving any money in the longrun. Hypothetically, if you were saving any fuel (which you probably aren't) the fuel savings are at a cost of increased wear on brake pads and rotors - causing premature wear and earlier replacement. These costs would far outweigh any negligible fuel savings. And then when you consider the safety implications it just seems "silly".

As far as it being bad for the transmission. I have heard myths on some vehicles that the transmission doesn't receive active lubrication (fluid) circulation - which is harming. However, I think this is a myth (as in not true), especially in a mercedes-benz. But it COULD be true - I really don't know.

I don't think its unsafe to shift from neutral into drive at idling RPM's, or causes EXCESSIVE wear and tear on the transmission. But it still causes more wear than keeping the vehicle in drive. So imagine you did this 5 times in one trip (putting it in neutral while coasting). If you kept it in drive the whole time, when you got home, would you think twice about shifting your car from park into drive 5 extra times? It is probably a little bit different, as the wheels aren't moving - so its a bit of a harsher transition, but you get the point.

Bottom line is this: You won't save fuel, you won't save money, it is borderline dangerous and negligent - DON'T do it!

 

I used to actually get pretty good mileage on highways at night if if blasted immediately when i got on the road to 120 or 130 mph then put it in N and let the car coast until it slows by itself to the speed limit (takes a surprisingly long time) then slid it back in D and put cruise control on. However, I think this may have hurt my transmission as it did go into limp mode a couple times and since I stopped doing this it hasn't gone back into limp mode again.

 

What's "limp" mode?

 

what I mean by limp mode is that the car gets stuck in a gear and when i try to manually change gears it shows on the dash that I am changing gears but the car still stays in that gear. When I put my foot down there is also no kickdown and takes a while to get going and if I put it in N and back in D the transmission stays in N. Only way to fix it is pulling over and restarting the car. I think doing this kind of driving may have caused it

 

Looking at the "Towing" section of the manual it stated that the towing speed must not exceed 50km/hr or the distance exceed 50 Km .

Also on page 116 Neutral "Do not move the selector lever to N while you are driving . The auto-matic transmision could other-wise be damaged. "

This in my mind says that putting the car in "angel gear" is not a good move.Also you have a better chance of being one with this proceedure as you have less control of your vehicle in an emergency.

If you want better economy best buy a diesel !!

 

Do not place the transmission in Neutral while the car is moving.

 

Interesting topic.
I noticed that the cabs in Hong Kong almost always put the transmission in neutral while waiting for lights, etc. One of the drivers told me it was to save a little fuel and reduce heat build up in the trans. The fuel savings make sense as the engine does not have the pressure from the torque converter present while idling. The heat reduction would also help them a lot on hot days on those steep San Fran like hills.
As far as slipping it in neutral while coasting, jctevere was correct that you actually use more fuel. When coasting, the cars momentum keeps the cylinders turning and not the fuel/air mix....air is the only thing going through which also helps to cool the engine. When idling while coasting, you are injecting fuel/air to keep the egine running. Not to mention it is dangerous and the loads slapping around in there when you "slip" in and out of D and N at speed are not something you want happening all the time.
I would speculate that it wouldn't hurt at stop lights etc, but I wouldn't do it at speed.

 

Would the trans oil pump stop if the atx were coasting down a long, fast hill? I think not, but don't remember.

I recall an old test where mercedes always used wot to come up to speed and short shifted, to get better mpg's by avoiding pressure drop across the TB, and other stuff.

.

 

Although in Manual it states that shifting in to N can hurt tranny, i think it really putting it back in to D if the car is moving is what kills it.
Getting it out of D in to N disengages the gears and that should be fine. its engaging back while moving when transmission being set for 1st or 2nd gear when it really may need to be in 3rd is the problem.

 

I used to do this on my old C43 and it coasted me $4800 on a new trans, so unless you have money to waste, I suggest you don't do it!

 

Nice word play!

 

As stated before, it will prematurely wear out your transmission since automatic gear boxes were not meant to be put into neutral as frequently as manual ones. The potential damage/premature wear of your gearbox will far outweigh the savings on gas. But more interestingly it is illegal in most states to coast in neutral but you might want to check your local laws about this issue. There were extensive discussions about this on the gas savers forum: http://www.gassavers.org/archive/ind...ng/t-9946.html

 

you use more gas. engine braking does fuel cutoff and uses no gas. if you put in neutral, your engine needs gas for that duration.....

 

IIRC, the reason for not shifting to Neutral while rolling is because the ATF pump stops working and can cause trans damage. Although I also seem to recall this is not true for the 722.6 5-speed, that will continue pumping. Not sure about 722.9.

The bottom line is the car can coast in D without using gas. It would use gas to coast in N. Makes sense to leave in D.

 

Placing the transmission in Neutral while moving disables all the transmission protection such as steering extra fluid to the TC if an overheating situation exists & steering more fluid via the cooler circuit etc. Don't do it.

BTW - Benz transmissions keep the oil pump running in neutral unlike older NA transmissions.

 

I don't think the gas savings possibly achieved by the described technique will be enough cover the future tranny repairs and other wear and tear caused by it.

 

guys if you didnt read my post, leaving the car in drive will use no gas when u coast to a stop due to the fuel cutoff. if you put in neutral, you use more gas and possibly do more damage.

 

...subscribed to read later...

 

End of story.

 

Flaccid Transmission.

 

Not only that you are not saving, but your are spending much more.

Using the engine brake cuts off the fuel injection. You can even notice that if you drive down on a steep slope on D and without applying throttle, the car temperature will go down which means that there is no combustion (also take a look at the current fuel consumption in the dashboard which shows 0). While you are on neutral it shows the maximum possible (20l/100km on my car).