I came across an idea that seems brilliant and, because it's me, is probably not. Here's my thought process:

Background: I drive a '99 Corolla VE with a 3AT (sadness!--just waiting till i can afford a Fit). I drive mostly in the city with a few highway jaunts. I'm averaging 26-28 mpg, which isn't terrible. But it could be more. Oh, and the awful thing is, I have no tachometer (so none of my measurements can really be scientific).

So I'm considering starting to put it into neutral when I'm idling at a light and when I'm coasting, then putting it back into drive when I need to accelerate or start from a stop. (This is quite easy for me, since I don't have to push the button on my shifter to switch from N to D). I tried it a couple times on my drive home, and here are my findings:

1) I can coast a LOT farther! I think this is because with my foot off the gas with the car in drive, I guess there's some engine braking going on? Whereas with the car in neutral, there's no resistance and pure momentum...
2) The revs are lower when coasting. Obviously, since the engine isn't trying to do any work.
3) I have no way to know this empirically, but I'm guessing that the engine revs lower at idle in neutral? I know that to avoid a slight lurch I have to rev it juuuuust slightly to put it in drive at a standstill.

The real question, though, is whether or not I'm going to destroy my transmission with this. I've already gathered that I need to rev-match to put it into gear when driving or even at a rest. But I'm concerned about the life of my (already 12-years / 115k mile) transmission. Any idea if it'll be worth it? It's way too soon to tell if I'll get any better FE by doing this (but if it's not crazy dangerous for my car, I'd be willing to give it a shot).

Oh, one last thing... a friend of mine seems to think that I could do damage to my transmission/engine by braking hard while coasting with the car in neutral. I can't see any way for this to happen, but I thought I'd throw it out here... is my friend crazy?

Thanks in advance.

~MBC

 

yikes, this whole time I was answering as though this were about an MT. so I had to erase my post and start over.

1 & 2 are related to the same thing... the torque converter.

In an AT, there's the TQ, in place of the clutch in an MT. The torque converter, when unlocked, mimics a slipping clutch... it means that some/most of the spinning on one end does effect the other end. And usually, that's the engine crank going to the transmission.

When you coast, the engine isn't doing work per se, but instead, "being worked" by the transmission. The spinning of the wheels actually transfer BACK to the engine through the TQ. Because the TQ never mimics a completely disengaged clutch, only slipping or fully engaged! So, it'll "push" the engine a little bit (increasing the revs a little over idle).

When you're idling at a stop... hum... having thought about this... i'm not sure anymore. but when you are giving it the extra rev at standstill, is your other foot on the brakes? I think the lurch is really the brakes holding the wheels down, while the transmission is trying to torque everything forward. With your foot off the brakes, it simply moves forward (albeit slowly). Considering this, it should actually rev lower while IN gear, because of the TQ slowing the engine down, since the other side is the transmission, which isn't moving.

As for damaging the transmission... well, yeah, the truth is, it's VERY hard to rev match an AT with the engine, since you don't have direct control over which gears to shift into. Nor do you have the ability to control the TQ the way you could control an actual clutch. I have on a few rare occasions, managed to get lucky, but most of the time, when I neutral coast or even neutral idle, I can feel the jarring of the transmission going into gear.

But simply braking while in neutral isn't in itself going to damage the transmission. It's only the shifting that is wear and tear.

 

Leave it in gear and coast. It shuts the injectors off, till you cross the fuel cut threshold, which is very low. I know in the MT's it is less than 1000rpm.

Do not shift in and out of neutral in your auto. Yes it can be done. Just don't.

 

What is the threshold? I don't mean the actual rpm, but what it does.

I was messing around with my drive the other day, trying to see if the fuel usage drops in gear or in neutral when coasting. I have the ultra gauge plugged in... and it shows higher usage in gear.

At idle engine speeds, it shows 0.2 gallon per hour. and when neutral coasting at 40 mph, it is the same... but if I have it in gear, its least 0.35/0.4 or more. I've tried using lower gears and higher gears... none show as using less, according to ultra gauge.

that leads me to one of two conclusions...

1) it's using a minimum level in gear, which is higher than idle.

2) it is using less by actually shutting it off, but ultragauge is unable to pick it up, and is basing fuel usage on something else.

... now that I think about it... ultragauge was reporting that I had 0.77 gal left in the tank when I went to pump gas, which should mean that I used up something like 9.8 gal. but on the first click, I had only pumped 9.3 gals.

hum...

 

put it in drive and leave it. automatics are very fragile and the more you play with it, the more quickly it will wear out

 

Some extreme hypermilers will actually shut their cars off to coast. I'm not sure how they get around the steering wheel locking up. I read an article recently and If I can find it I will post a link. The guy in the article was the king of hypermilling. No a/c, No rolling the windows down, Riding a foot away from other cars and trucks, not stopping at stop signs...this guy was dangerous and nuts, but got great mpgs.

I get frustrated sometimes with the engine breaking in the fit when I want to coast, but I'm not taking any chances with throwing it in N.

 

Braking hard while in neutral would only really put stress on your axle, and minimal stress at that since it's not radially large like a wheel. Neutral means that your transmission is disengaged from the engine, so there's really not much you can do to the engine.

However, braking hard does do a number on your chassis, since your car body is still trying to move forward while you're bringing it to a stop. A good deal of flex occurs unless you've stiffened it up with race parts.

The MPG gains between switching from drive to neutral in an automatic are extremely minimal. It's a difference of maybe a hundred or so RPMs. You'll get much better gains from engine braking to a stop as if your brakes were broken (engine braking shuts off your injectors while coasting in neutral still uses gas to idle the engine).

 

There are many people out there that take part in a driving technique called hyper miling, it uses many of the techniques you mentioned about coasting in neutral. This will not do damage to the transmission the only time you putting unnecessary wear and tear on the tranny is when your moving and you shift back in to drive. But this shouldn't cause any damage unless done while your holding the gas or something of that matter.

As for rev matching, i have never attempted to rev match my 07' sport from neutral into drive, it would probably lurch the car or lag on the shifts and give you a bumpy ride BUT I constantly rev match daily shifting with the paddles, blip the downshift paddle, blip the gas at the rite time and you can rev match while downshifting just as perfect as a manual car.

 

You might not be "destroying" it but you're certainly adding to the wear which will eventually lead to mechanical problems sooner than they would otherwise happen.

 

Thanks everyone for your thoughtful and well-thought-out replies! I guess this is a really silly way to try to boost FE, huh? Stupid "moving parts" BS...

So much for my "midnight-thinking"...

~MBC

 

The best/easiest way to boost FE is to increase your following distance so that you never have to brake hard. Accelerating slower would also increase FE, but that makes driving less fun.

 

It's also a tiny bit dangerous and a tiny bit illegal in many states, driving in neutral.

 

Putting it in neutral while at a stop-light is fine, and I don't see any problem with it. You will increase your mileage, because the engine is doing less work. With an automatic that has a torque converter, there is a viscous coupling which is constantly being turned by the motor. Placing the transmission in neutral allows the transmission input shaft to spin freely of the gears, and therefore allows the engine to spin more freely, reducing the required amount of fuel input.

Are you certain? It's my understanding that injector shutoff is for manuals only.

I've done a similar thing when I had an automatic, placed it in N to coast long distances. Your only real danger is in the possibility of throwing it into R while moving. I had a ratchet shifter so I wasn't worried about it, as long as you aren't afraid of putting it in R you won't have any problems or cause any perceivable additional wear.

 

Most every car I've ever driven rev's higher in neutral than in drive. The lurch you're describing is the sudden load you're placing on the engine. I'd guess that by revving you're providing a bit of energy necessary to smooth that transition out.

 

i'm betting with an AT, you'd likely ruin your transmission in the long run.. transmission will have a hard time engaging the correct gear matched to your current coasting speed.... you can do this without any major side effect with an MT though..

 

In our cars, the AT gear selection is controlled by the computer, unlike older models where the AT was entirely self contained.

I can speak to this from some personal experience... I had a commute with a several mile long downgrade where I could slip it into N at 67 and coast for a couple minutes, speed would drop slowly to about 63. Shift back into gear was smooth without me doing anything beyond moving the shifter.

I would do this not for the miniscule fuel savings, but just because I preferred the relative quiet... This was before I added sound control materials....

 

Leave it gear and drive. It will not make a big difference in milage. Honda ATs are not that great. My Accord died twice, and was driving "Normally".
Even the MT will be less hard on the Transmission doing this, but it will put more strain on the throughout bearing and clutch. Even though it is not overly safe to coast in neutral, I will do it on really long down hills, when there is no traffic.