Coasting downhill in my car in neutral: how bad?

Factual Questions
21

I have a manual transmission Honda Insight.

I push in the clutch every time I go down a hill. This really does boost the overall gas milage of the vehicle. Traveling a 1/2 at idle speed = less gas than the same at 3K rpms.

Going down the hill with speed does not cause the engine to heat any higher… at the idle speed water is still moving thru the engine, but the air rushing thru helps cool things off.

Of course, if I gain to much speed I have the option of releasing the clutch and having the gears slow me down. So I do not wear the breaks down to much.

The ins and outs of all this for an automatic transmission I do not know.

22

With an automatic transmission, the engine disengages when you take your foot off the gas. So, in effect you are in neutral even the shifter set to “D” if you are coasting down a hill. So if you slip the shifter to “N,” it will have no effect on your car’s drive train.

The only effect will be that if you want to speed up, you’ll have to put the car back into gear.

To test this, keep your eyes on the tach when you drive. Every time you take your foot off the throttle, the RPMs will drop to idle the same as if you throw the clutch out in a manual car. No matter how fast you’re going.

Just MHO.

23

One thing I did not see mentioned about shutting off the engine when coasting is the loss of power steering.

24

Country Squire, what kind of car is this? Make, model & year.

25

LET ME REPEAT… I am not talking about shutting off the engine. I have no intention of shutting off the engine. I promise not to shut off the engine.

Okay? Thanks!

26

I find this hard to believe, given that automatics will drive forward if you aren’t actively pressing the brakes, but maybe I’m missing something…

27

You are not the only one posting in this thread, Squire. We’re not just talking to you. OK?

28

It seems to me that whatever small gain in fuel mileage is not worth the added danger you’ve put yourself in by coasting down grades. The engine’s compression is a major factor in keeping your speed down, along with the brakes. That’s why you downshift to lower gears going downgrade, so the compression can keep the speed down enough so you don’t have to use the brakes continuously. Relying on brakes alone is an easy way to overheat them and boil the brake fluid. If that happens then you have no brakes at all until they cool down, which will probably be after you’ve plowed headon into that ditch or the truck coming up the grade.

29

The Car Talk brothers on coasting downhill.

30

Two words: ASK GARP.

31

Now that I think about it, you’re right. Nevertheless, if I am driving my wife’s car (an automatic) at 60 mph on the highway and I see traffic slowing up ahead, so I take my foot off the gas, the tach goes right down to 800 rpm – idle. The only way this is possible is if the engine is disengaged.

So perhaps the automatic transmission disengages if the car’s motion would cause the engine to speed up, but not vice-versa.

32

Lucwarm, the reason the engine speed drops to idle when you take your foot off the gas is because the engine output and transmission input are not connected mechanically; there is a fluid-filled torque converter on the front of the tranny. The TC is connected to the engine’s crankshaft via the flywheel and spins with it. Inside the TC and fixed to it are fins that act as a pump, flinging fluid outward. This fluid enters another set of fins, a turbine, and causes it to spin. The shaft of the turbine is connected to the tranny.

33

There are lots of variations in transmissions folks.

Some will not disengage ( free wheel) if you take your foot off the gas.

Some will free wheel in D but not in 3’rd and is usually a slight down shift in gearing.

Some automatics always (free wheel) and they are harder on brakes because they have no engine braking.

Some transmissions have overdrive as an selection and those are usually a (free wheel) condition. In overdrive that is. But some were not Free wheel even then.

There is no difference nor as much strain on a tranny by shifting from neutral to drive while coasting at 50 MPH than there is downshifting from drive to second to accelerate out of a dangerous situation ala James Bond. In a properly set up tranny, you will drop two gears by just flooring it from cruising to full on the floor. trannys get plenty of fluid movement at idle to take care of a neutral to drive shift at 50.

The largest damage of trannys is people who drive around in overdrive while in city traffic where the tranny is constantly shifting up and down.

One reason for more Free Wheeling transmissions these days is the fact that cars are so small and light. An 1980 Caddy or Buick or Chevy Impala are not as likely to have free wheel in the tranny.

Just because you are young and have not driven all models is not a problem, but there are older and different cars on the road that you all seem to be totally unaware of. But I admit that is hard to say for sure of the last 5 years or so, I need to read the data plate to tell them apart. Now ask me about anything from 1948 to 1965… Bawahahahaha

About coasting… If the engine is not turned off, and the speed does not build to a point where you are speeding (against the law you know) then putting an older transmission in neutral is no more dangerous than simply driving down the hill in the free wheel equipped cars, or using the average ‘overdrive’ in most cars so equipped.

The boogie man about needing sudden acceleration is just scardykat people wringing their hands. If you can’t remember to leave the engine running, do it in bumper to bumper traffic in CA, can’t shift back into drive, then, you should not do it at all… You are to stupid to be in a car in the first place. ::: sheesh ::::

34

But, if this is for the engine is not off folks, the torque converter is still turning the pump. The converter body, not the input shaft, turns the pump. So as long as the engine is running the pump is moving fluid and since there is no pressure on the parts there is no need for high fluid pressure.

35

I would believe that this car has a coast program in the computer that lills most of the fuel to the engine during coast conditions. If it does, then you are forcing it to idle fuel when doing what you talk about. I bet you are using more fuel!

36

For that matter, when you’re in a Honda Insight, and you’re driving around trying to strategically FURTHER increase the gas mileage, you might just be beating a dead horse… Did you lose a bet? <=

37

Did anyone mention DON’T SWITCH OFF THE ENGINE WHEN YOUR CAR IS MOVING!

38

wow, there is so much incorrect information in this thread. but i guess that is normal when it comes to cars. everyone thinks they’re an expert. ill sum all the points up in one post, i saw a few posts with good information but they are surrounded by wives tales and just plain ignorance.

Modern cars have lockup torque converters… when youre going down the highway at 70mph and let off the gas, the rpms will NOT drop to idle. When a torque converter locks up, the car is essentially like a manual transmission, there is no slippage, it is a direct mechanical link. This usually happens above 45mph as long as overdrive is turned on, and you are not at wide open throttle.

With that said, no, it wont hurt. coast down a hill, big whoop dee doo. Some of you are partially right. Towing an automatic RWD car with the rear wheels on the ground is a bad idea if youre going long distances. If that is the case you need to remove the driveshaft.

But since we’re not talking about towing… as long as the engine is on, the front pump in the tranny will be getting fluid. AND SINCE WE ARE IN NEUTRAL AT THIS POINT, there is no need for a high line pressure, because the transmission is in neutral.

having the engine go from idle (probably5-800rpm) to say 2,000 or so is fine for the engine but it could eventually cause problems for the transmission, since you dont have a performance car or anything, you would most likely NEVER NOTICE. if you want to be safe and you have a tach, get the rpms back up to ~2000 or so before putting it back in gear. its similar to what you would do if you had a manual tranny… 80mph on the freeway, you pop it in neutral going downhill… you wouldnt justput it back in to gear and pop the clutch, youd try to match the engine rpm with where it should be. same principle.

39

Item # 15, 389 that the SDMB is FULL of folks that do not read even short therads before swelling with pride form seeing their post up… ;j \

::: sheesh ::: :smack:

40

Get a clue Gus.