Coasting in gear or neutral - most efficient?

[hush]

In such a situation if you knock it out and back into gear it 'might' bump start then engine back into life.

 

If it doesn't restart just having the engine turning over will still generate vacuum in the brake booster to give full power to the brakes.

It'd also keep the power steering pump spinning so the steering doesn't get heavy.

 

The problem was the ignition had failed. So the engine wouldn't restart.

You wouldn't believe how quickly a car slows down when it is in gear on dead engine. That's not too cool when you are in the outside lane of a motorway at rush hour and trying to thread through a line of trucks that are doing around 60 in the inside lane.

I thought that I had done OK when we got to the hard shoulder. Unfortunately it ran out before the car stopped and I hit a bridge parapet at about 15 mph.

[Justin Smith]

I coast around corners/junctions in gear, and down hills in neutral. I also have a bad habit of putting the car in neutral too soon before stopping.

I remember my driving instructor telling me that coasting is bad practice in any form.

 

Me too, mainly because I drive in the highest possible gear. If one is literally to be in gear till an instant before one is stopped, as one slows down, one would have to change down at least once and possibly twice before one stopped. I have read "it`s dangerous to have your car in neutral", but, to be frank, if you can see the light is on red and you`re already slowing down for it, maybe even already braking for it, under what circumstances* is it "dangerous" to have the gearbox in neutral ? How often would you have to speed up again (and if you`re in a high gear you wouldn`t be able to accelerate very fast anyway) to avoid a dangerous situation ? Bear in mind you can have the car back in gear within a fraction of a second anyway. I`ve thought about this a lot and cannot think of such a scenario.

 

* This specifically excludes if you`re going down a steep hill, I always go down steep hills in a low gear

Edited December 17, 2016 by Justin Smith

[Cyclone]

You should have an area to yourself Justin.

"Justin's Driving tips"

Where you pass on your expertise, such as not changing gear and instead moving to neutral, not changing lane because it's dangerous and how to count the many makes of car, whilst also concentrating on the road.

[ANGELFIRE1]

A little different I know, but I would love to stick some of the characters on here in a heavy truck. You would know within a mile why you should NEVER EVER coast out of gear. It is tantamount to commiting suicide, or worse, killing someone else.

 

If you are that desperate to save fuel, buy the smallest engine in the smallest car, then you could drive it correctly and safely.

 

Angel1.

[Justin Smith]

Me too, mainly because I drive in the highest possible gear. If one is literally to be in gear till an instant before one is stopped, as one slows down, one would have to change down at least once and possibly twice before one stopped. I have read "it`s dangerous to have your car in neutral", but, to be frank, if you can see the light is on red and you`re already slowing down for it, maybe even already braking for it, under what circumstances* is it "dangerous" to have the gearbox in neutral ? How often would you have to speed up again (and if you`re in a high gear you wouldn`t be able to accelerate very fast anyway) to avoid a dangerous situation ? Bear in mind you can have the car back in gear within a fraction of a second anyway. I`ve thought about this a lot and cannot think of such a scenario.

 

* This specifically excludes if you`re going down a steep hill, I always go down steep hills in a low gear

 

Cyclone`s "answer" :

 

You should have an area to yourself Justin.

"Justin's Driving tips"

Where you pass on your expertise, such as not changing gear and instead moving to neutral, not changing lane because it's dangerous and how to count the many makes of car, whilst also concentrating on the road.

 

So now I know why, technically (as I asked), it is dangerous to take your gearbox out of gear whilst slowing down to a stop. In fact you`ve even given me a real world example of why so. Thank you, as always for your, objective, helpful, and never personal, answer.

 

I should have known better than to ask any question about motoring, I can`t ever remember any motoring thread, whether started by myself or not, that hasn`t degenerated into negative personal comments.

If I`m such a poor driver, why have I not had an accident in 25 years ? No don`t answer that, it`ll not be something I, or most others, want to hear.

 

---------- Post added 17-12-2016 at 11:05 ----------

 

There appears to be a little confusion on this thread, between coasting out of gear with the engine running, and coasting out of gear, with the engine switched off. The former should ideally not ever be done (though personally I can`t really see why it`s dangerous if you`re coming to a stop anyway), the latter should never be done.

Edited December 17, 2016 by Justin Smith

[ANGELFIRE1]

Cyclone`s "answer"

 

[/color]There appears to be a little confusion on this thread, between coasting out of gear with the engine running, and coasting out of gear, with the engine switched off. The former should ideally not ever be done (though personally I can`t really see why it`s dangerous if you`re coming to a stop anyway), the latter should never be done.

 

 

You would fail your driving test if either of the above were done.

 

Angel1.

[Cyclone]

How is it a personal attack to list all the terrible advice and opinions you have about driving Justin? They're your opinions, I was just compiling them into one handy list of how not to drive.

[Justin Smith]

How is it a personal attack to list all the terrible advice and opinions you have about driving Justin? They're your opinions, I was just compiling them into one handy list of how not to drive.

 

yeah, yeah, yeah.......

But back to the subject in hand.

Maybe Cyclone, or anyone else, could help me with this one. I have frequently wondered what situation on the road would actually be dangerous if you had your gearbox in neutral (engine running). In fact we were just talking about it at work and could not think of any scenario where that would be the case. I managed to come up with one (and I`m not being facetious, this is the only thing I can come up with) :

 

You`re driving along at 30mph, you see the light change to red a few hundred yards ahead, so you knock it into neutral and let the speed drift down. Suddenly a helicopter falls out of the sky right in front of you, and you`re too close to brake and avoid it. I should add here that there`s wall on your left and a line of cars on your right so you can`t steer out of the way. If your car had been in gear (and it would have had to have been a low gear to give you significant acceleration, and you had the nerve and presence of mind to do it, pretty unlikely, but there we go) you may just have been able to accelerate out of the way before the helicopter crashes in a fireball just behind you.

 

That`s it, I can`t think of anything else. But even on this one it doesn`t really work because even if you were in neutral you`d be able to get it into gear within a fraction of a second anyway.

 

---------- Post added 19-12-2016 at 10:24 ----------

 

You would fail your driving test if either of the above were done.

 

Angel1.

 

Would you not fail your driving test for parking on the kerb ? Something nearly everyone does, and (so long as there`s still room to get a wheelchair and/or large pram past, and it`s not next to a junction) it isn`t even anti social.

Edited December 19, 2016 by Justin Smith

[L00b]

But back to the subject in hand.

You'll come to a stop earlier if your car is in gear, due to the mechanical resistance of the drivetrain (power is bled from car to engine through drivetrain, instead of applied from engine to car through drivetrain: forces are loaded onto the opposite side of the gear teeth), which is additional to the brakes themselves (brakes are external to the drivetrain).

 

This engine braking action will also be far more evenly distributed about the car (essentially co-axial with the longitudinal axis of the car) relative to only the brakes on 4 corners (particularly so once you factor in any uneven tyre wear, uneven tyre pressure, uneven pad wear, any warp or other imperfections in discs <etc.>) and so improve (as in maintain better-) control under hard deceleration.

 

The lower the gear, the higher the load on the engine and drivetrain, so if you want to "coast", stay in the highest appropriate gear and just let go of the go-faster pedal and, if you want to brake, drop gears progressively as the car bleeds speed.

 

The other way around, driving on, this is basically how cruise control works: highest gear for the speed and load (typically, road gradient and laden weight) and 'blip' gas on/off with adjusting stoech mix as she goes.

 

It's really not rocket science, but simple physics and mechanics.

 

No helicopters were harmed in the redaction of this post

Edited December 19, 2016 by L00b

[TimmyR]

I was told that waiting in traffic with the vehicle in Gear and Clutch disengaged is a major contributing factor to premature clutch release bearing failure as this loads the clutch release bearing excessively and shortens both the bearing and clutch assembly life drastically.

So, driving with the car coasting and the clutch out would possibly give the same result.

 

<Dubious face>

 

---------- Post added 19-12-2016 at 11:44 ----------

 

You'll come to a stop earlier if your car is in gear, due to the mechanical resistance of the drivetrain (power is bled from car to engine through drivetrain, instead of applied from engine to car through drivetrain: forces are loaded onto the opposite side of the gear teeth), which is additional to the brakes themselves (brakes are external to the drivetrain).

 

 

This engine braking action will also be far more evenly distributed about the car (essentially co-axial with the longitudinal axis of the car) relative to only the brakes on 4 corners (particularly so once you factor in any uneven tyre wear, uneven tyre pressure, uneven pad wear, any warp or other imperfections in discs <etc.>) and so improve (as in maintain better-) control under hard deceleration.

 

The lower the gear, the higher the load on the engine and drivetrain, so if you want to "coast", stay in the highest appropriate gear and just let go of the go-faster pedal and, if you want to brake, drop gears progressively as the car bleeds speed.

 

The other way around, driving on, this is basically how cruise control works: highest gear for the speed and load (typically, road gradient and laden weight) and 'blip' gas on/off with adjusting stoech mix as she goes.

 

It's really not rocket science, but simple physics and mechanics.

 

No helicopters were harmed in the redaction of this post

 

Quite a lot of nonsense in this post:

 

"You'll come to a stop earlier if your car is in gear" - usually you come to a stop when you want to, the only time you want it to be as fast as possible is in an emergency and in that circumstance the limiting factor on how fast you can stop is the grip of the tire in the road. How the wheel is retarded is irrelevant, whether its your brakes or your engine. Modern brakes have more than enough power to make the wheels skid.

 

"This engine braking action will also be far more evenly distributed about the car" - this is total nonsense. assuming its a two wheel drive car, the engine braking effect will only be applied to 2 wheels so will be using half the available grip and will also wear the drive wheels more.

 

The answer to original question was answered correctly in the second post. Coasting in gear is more efficient as the engine is not idling

 

There is a historical reason that you should change down through the gears when braking and that is to mitigate the risks of brake failure - modern brakes don't really fail so its less of a concern.

 

The advanced driving instructor will tell you that you should apply brakes and only change down when you next intend to go. Brakes are for slowing gears are for going. You should never coast out of gear but you should not be using the gears to slow as is suggested above. The reasons are:

 

Increased wear of the clutch which is expensive to replace compared to the brake pads.

The distraction of changing down when you should be focussing on braking.

As you change down the engine has to use fuel while the clutch is in to keep the engine going so you waste fuel.

more balanced (not less as the above post suggests) as you are using 4 wheels to brake not 2 (assuming 2wd)

Edited December 19, 2016 by TimmyR