so I had done some keyword searching and google searching but it seems as if I couldn't find a thread on this from here.

basically i'm curious as to who throws their G in neutral going downhill and whether it's proven to be detrimental to the tranny. I'd imagine if you rev-matched before throwing it in drive or were slowed down significantly it wouldn't really matter but I always like getting some information from the board.

Any thoughts?

- Bob

 

That is a good question. I do this a lot if i know the hill will keep me at the same speed or faster. But before i put it back in drive a rev it up to match the rpm to the speed i am going. Would like to know more on this.

 

I'm really not sure why you want to do this. There is no significant gas saving in this maneuver and you lose the effect of engine braking. Aside from the fact that putting the car into neutral and then back into drive, gives you one more thing that you have to distract you while driving, by coasting in neutral you have basically given up full control of your vehicle.

So, why bother?????

Mike

 

I do this all the time. My tranny runs great. Our computers on the G are smart and can figure this out. Everytime I do this the engine just revs up automatically and the transmission goes into gear. I don't even try to rev the engine up and put it into gear. That might be problematic because if you rev it up too high the gears may catch and cause a neutral drop and that is bad for transmission.

I've accidentally done that to the Lexus ES350 though, and you can feel a big jerk when you rev up the engine and put it into gear while coasting.

I don't know if there's any significant gas savings or not, and you do give up control, but I'm used to it. I also don't like putting my feet on the brake for long periods of time at a stoplight, so sometimes I change it to neutral.

You only get a significant gas savings by accelerating and decelerating smoothly.

 

It's been proven that doing this does not save any gas at all, and is in fact pointless and potentially dangerous. Has to do with the engine using it's own momentum to keep things moving while in Drive, and it doesn't use any real amount of gas in the process. Plus if someting happens and you need to accelerate for some reason you can't.

Some auto mag did a test of MPG, comparing coasing in N, using A/C vs. windows down, etc.

 

i want to thank everyone for the responses, but i'd respectfully disagree with people who are saying you lose all control of your car while coasting in neutral. clearly that's an exaggeration since your brakes and steering obviously don't cease to function when you're in neutral.

funkydrummer raises a good point that if you have the need to accelerate you're SOL, however, if you're going downhill i'd imagine your need is more to stop than anything. either way, in foul weather this would not be the way to go, since you obviously lose your AWD once in neutral (of course you also lose AWD when you're braking too, so even that can be detrimental).

i don't know what car magazine you were looking at (i'll assume the popular mechanics article i googled is what you're referring to though) so i'm not judging their credibility, but i would throw out there (for conversation's sake) that mythbusters "proved" that driving with the windows down saves more gas than using the AC. If you google "annotated mythbusters episode 22" you'll find the synopsis.

mikeb2 - i wouldn't care about engine braking if i was using my hydraulic brakes. but i see where you're probably going is that it's increased wear on your pads so will you really save money over time.

anyways, after reading the popular mechanics article i just mentioned i'm happy agreeing there is no benefit if you're taking your foot off the gas while going downhill. i did just read a brief article on hypermiling though and there's nothing new there i haven't heard, like turning the engine off at lights, which i don't think i'm willing to bother with. oh well. guess i'll just have to keep driving sensibly to save that gas

 

I like to think i am saving gas. Your foot is not on the peddle and the RPM is down to 800, vs. gas on the peddle RPM up to 3200 if your going 75mph. And if i am going down hill for 4+ miles and going about 70-90 (90 when i start pushing the break peddle a little bit) that is gas savings right there. And your MPG goes up more. But we are not talking about Gas, control, and what ever else. We are talking about the tranny. How does it effect the Tranny? if any?

 

hey bluedevils95 - sounds like our trannys should be just fine based on the computer rev-matching the engine for us.

and in the end i guess the issue is that i didn't see a study myself that shows me the empirical data of how much gas the car uses in neutral vs. coasting downhill in gear but after reading the pop mechanics article i like to think our cars are smart enough to know to cut the injectors down.

i may do an experiment myself to let you guys know....we'll see how committed i am

 

I notice when i was going down a 8 mile hill that my MPG went up going 85pmh

 

^^ THIS IS TRUE

And additionally, for anyone who still does shift to neutral, I wouldn't do it for long time intervals and I would definitely make an attempt to rev match before shifting back into gear.

This whole 'coasting in neutral' question brings up a consideration of maintaining proper hydraulic pressure throughout the tranny since the torque converter is not spinning at the right speed when the engine is just idling.

Picture yourself in gear and at a complete stop on a bit of a hill where, if the grade is just sufficient, you don't even need to hold the brakes since the torque output of the tranny at that engine speed JUST matches the force of gravity pulling the car. The ability to make a balance such as this is evidence that there is not enough hydraulic pressure generated by the torque converter at those rpms to power the car forward against even modest resistance. You have to spin the engine much faster to provide sufficient pressure within the tranny to overcome gravity and accelerate from there.

SO, let's say after you've coasted downhill for a nice 4+ mile grade, and your hydraulic pressure throughout the tranny has long since bled off, you are now going to do WHAT?? . . .slam it into gear and wait for the hydraulics to pump up and equalize to the appropriate level? !! UGHH . . I don't think so; I think you want to get that torque converter spinning fast enough to get pressures up to par with the speed you're going BEFORE you pop it into gear. Unless you can prove the engineers who designed our cars intend for the rev-matching function to work with the tranny at other than steady-state hydraulic pressure levels (ie. from idle vs. normal driving pressure levels).

One last thought: consider the restrictions on towing most automatic cars with the rear wheels on the ground; WHY do you suppose that's not recommended?

 

vqsmile, i think i'm reading your last sentence wrong or something....do you mean towing automatic FWD, RWD OR AWD cars with rear wheels on the ground? i mean obviously on a FWD it wouldn't matter, and on a RWD you obviously wouldn't want it in park, but i see plenty of motor homes towing their jeeps behind them with 4 wheels on the ground, and most of us go through belt driven car washes in neutral so i think im missing where you're going with that...and just as a clarification, i'm not disagreeing i'm just not seeing what you're trying to get at for some reason.

and good point about the transmission.

 

If it does harm, i dont want to do this anymore.

 

I think it comes down to an extremely negligible savings in gas vs. potential for accident or tranny wear and tear. I'd say if your 6mt, no prob., but with the Auto it just doesn't make since to do it.

And, yes I think it was the Popular Mech. article I must have read.

 

id never throw the car in N going down hill.

 

Whats the differencs from AT vs MT? Both can be in N. It would be the same