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-   -   Coast in-neutral vs in-gear: I've been challenged to show the numbers (https://ecomodder.com/forum/showthread.php/coast-neutral-vs-gear-ive-been-challenged-show-22841.html)

 reb01501 08-06-2012 11:00 AM

Coast in-neutral vs in-gear: I've been challenged to show the numbers

... which I thought would be easy to do by searching this forum. Well, it's turned out to be easier said than done. I've been through several threads where I expected to find the numbers to back up the logical conclusion that coasting in neutral loses less momentum and therefore results in lower fuel use. However, nobody seems to have done an A-B-A comparison, much to my surprise. The closest I've found is this post:
http://ecomodder.com/forum/showthrea...html#post41066

But he didn't do full tank comparisons of the techniques, which is what I think will be the best (only) way to convince the doubters. Has anyone done such a test and can post the results?

I know that the idea, once correctly explained, is so logical that testing it seems absurd, but some people take quite a bit of convincing.

 ksa8907 08-06-2012 12:17 PM

Why bother, it would be like convincing someone fire engines are red. Its obvious.

 euromodder 08-06-2012 01:06 PM

It's hardly possible to get decent numbers in normal driving, as you can't even start measuring from the spot where the coasting starts.

It'd have to be measured between a spot where one enters with X mph, and a place where one has to turn off at Y mph - that'd have to be reachable by coasting.

I started coasting shortly after joining ecomodder.
Coasting (even in neutral with the engine on) is what got me consistently below 5L/100km / over 47 mpg.
Up to then, I was avidly using engine braking, and only once in 5 years got below 5L/100km / over 47 mpg.

People are easily misled by the fact that their engine doesn't use gas when decelerating and letting of the gas pedal.
They don't realize they are also going to stop far, far sooner

 reb01501 08-06-2012 01:27 PM

I think what my challenger is after is more of a net mpg gain as measured at fuel-up time. Not the instantaneous mpg or gph during the coast. But after thinking about it, this would be hard to accomplish wouldn't it. There would need to be a test route that allows coasting and that is long enough to allow a measurable amount of gas to be pumped at the end of it.
The conditions would need to be similar for each test, which would be something like:
1. Fill the tank
2. Drive the route using in-gear coasting
3. Refill the tank and record data
4. Drive the route using in-neutral coasting
5. Repeat 3
6. Repeat 2 and 3.

Yes, I can see why nobody has done this.

 Duffman 08-06-2012 02:12 PM

Quote:
 Originally Posted by ksa8907 (Post 320347) Why bother, it would be like convincing someone fire engines are red. Its obvious.
Fire engines are not always red, but I agree with your premise that it should be obvious.

 euromodder 08-06-2012 02:18 PM

Let your challenger try it ;)

Nothing more convincing than first hand experience.
I too was reluctant to try it, but it works.

Despite the overshoots, even the first tries will yield results.
It will only get better when he gets more experience with coasting - i.e. ever more appropriate starting point / speed and thus better matching the coast to the desired speed at the next waypoint.

 Gealii 08-06-2012 03:14 PM

I did an AB test with this on 2 separate tanks it was roughly the same temperatures and traffic, and the same route the whole time. Coasting in Neutral resulted 23mpgs while coasting in gear resulted 20mpgs, even if by chance the cars do coast the same in neutral as in gear which i feel neutral coasts further your rpms drop in neutral resulting in better mpgs.

 user removed 08-06-2012 06:22 PM

Coasting in gear is not really coasting, since there is engine braking involved unless clutch disengaged in a manual). You can get hundreds of MPG coasting in neutral but if you are using engine braking you can engage DFCO and get infinite mileage for a shorter distance. Ideally you should be able to coast in neutral to a complete stop for the best theoretical mileage, but it just doesn't happen to most people in typical traffic.

I coast in neutral when it will not aggravate other drivers, then use engine braking when I need to slow down more rapidly.

regards
Mech

 reb01501 08-06-2012 07:56 PM

Thanks, but I've seen this reasoning hundreds of times in all the prior threads on the topic. And very convincing it is ... to me at least. It hasn't been until gealii's post that I've seen someone back up the reasoning with real numbers. You're all probably right that those numbers won't convince my challenger, but I do believe that you people have risen to the challenge. Thank you.

 Gealii 08-06-2012 08:04 PM

Quote:
 Originally Posted by reb01501 (Post 320418) Thanks, but I've seen this reasoning hundreds of times in all the prior threads on the topic. And very convincing it is ... to me at least. It hasn't been until gealii's post that I've seen someone back up the reasoning with real numbers. You're all probably right that those numbers won't convince my challenger, but I do believe that you people have risen to the challenge. Thank you.
also on those numbers the car had directional snow tires on it so the amount would have been smaller then a normal cars tires. I did not put the tires on the car they just came with the car and for something I only planned on having for 7 months which is about what i had the car for i didn't see a point in replacing the tires

 turbovr41991 08-06-2012 08:52 PM

I have been getting better gas mileage using DFCO in gear coasting with a pulse and glide technique. Better than what I was getting with engine running coasting pulse and glide. EOC is by far the best, but there are very few places n my commute where this would make sense.

 roosterk0031 08-06-2012 10:41 PM

Been messing with the Cobalt using engine braking, it goes open loop and gph goes from 0.31 at idle (ac off) to over 0.5 after releasing clutch with heavy engine braking, takes 5+ seconds for it to go DFFO if it does and then doesn't stay in it for long. Neutral coasting is way better, the factory gauge is only 2 digits, so wouldn't have known the difference without the SGII.

Edit: Paid a little more attention on the way home, if I crest a hill and leave it in 5th, after about 5 secons it goes Open loop, and up to 0.4 gph for about 5 more seconds, then goes into DFCO for about 5 more seconds (smallish Iowa hill, maybe 3% grade, 100' fall) then back to closed loop. Pretty sure Neutral Coasting would account for better FE.

 GRU 08-06-2012 10:59 PM

If someone is going to compaire the 2 techniques then they also need to change the driving style...example: if you start to coast in neutral at the same point of the road where you would when in gear then you are actualy using more fuel, so in order to use less fuel you can, and need to start coasting in neutral earlier than you would when in gear...
Its hard to explain but im just trying to create a scenario where someone is aproaching a stop sign or a red light

 ksa8907 08-06-2012 11:08 PM

Quote:
 Originally Posted by GRU (Post 320438) If someone is going to compaire the 2 techniques then they also need to change the driving style...example: if you start to coast in neutral at the same point of the road where you would when in gear then you are actualy using more fuel, so in order to use less fuel you can, and need to start coasting in neutral earlier than you would when in gear... Its hard to explain but im just trying to create a scenario where someone is aproaching a stop sign or a red light
Yeah, if I drive "normal" I would let off the gas about a quarter mile out, in neutral it would be more like a mile out.

 UltArc 08-07-2012 01:03 PM

I hope that person who said all fire trucks are red was joking.
That's not a constant, lol. Honestly, very little is constant.
The sky is always blue- what about a sunset or sunrise?
Gravity is always the same- it changes with elevation and location.

So maybe there vehicle, combined with their location, their traffic, driving style, and gerneral commute yield better results with that method.

I think it could be possible. I pulse and glidw from 74mph to 65mph, so I don't block traffic, and I only get to that speed by drafting. I was just thinking about taking a super compact car, say old Civic and putting a huge engine in, with at least a six speed, if the engine can run at very low rpms and maintain speed. I can often get 35-40mpg on my commute, but I stop at a light then I drop .1 rpm about every 5 seconds.

I don't know if I am clear, but my basic point is for ones own unique circumstances, it may be possible. But your point does appear to be more probable, if you use X fuel to get to a speed, and have Y friction, you travel a distance. If Y increase, auch as engine drag, over that limited area, mpg SHOULD be lower.

EDIT:

http://my.firefighternation.com/grou...age=2#comments

 Fat Charlie 08-07-2012 02:19 PM

I hope you're joking. The OP was asked to "prove" that coasting in neutral is more efficient than engine braking. If reb01501's challenger wanted data specific to his car and terrain then he shouldn't have asked someone to check the internet for data.

I've seen fire trucks in yellow, brown and woodland camo, but if my kid asks me what color fire trucks are I wouldn't say "it depends."

 UltArc 08-07-2012 02:54 PM

Quote:
 Originally Posted by Fat Charlie (Post 320505) I hope you're joking. The OP was asked to "prove" that coasting in neutral is more efficient than engine braking. If reb01501's challenger wanted data specific to his car and terrain then he shouldn't have asked someone to check the internet for data. I've seen fire trucks in yellow, brown and woodland camo, but if my kid asks me what color fire trucks are I wouldn't say "it depends."
Are you asking if my post was a joke? I am not super critical serious, but nothing was meant as a joke. And I didn't know he was asked to "prove" it, did he tell you that over a PM? I didn't see that anywhere, it seemed like a civilized debate, not so much an argument or anything hostile. And I'm not sure if they were asking reb about their car. It seems more likely they are only familiar with their own car, and they would expect others to have a similar experience.

I see, about your child. I don't have any children, but I'd say usually they are red, but you can paint it any color.

 reb01501 08-07-2012 06:53 PM

Not "prove" ... provide evidence to back up the assertion. There is a difference. I in no way believe that a few test result will provide absolute proof of the assertion. All it takes is one contrary result to disprove it. However, the lack of negative results is telling.

 jsatter 08-07-2012 08:24 PM

Quote:
 Originally Posted by euromodder (Post 320352) It's hardly possible to get decent numbers in normal driving, as you can't even start measuring from the spot where the coasting starts. It'd have to be measured between a spot where one enters with X mph, and a place where one has to turn off at Y mph - that'd have to be reachable by coasting. I started coasting shortly after joining ecomodder. Coasting (even in neutral with the engine on) is what got me consistently below 5L/100km / over 47 mpg. Up to then, I was avidly using engine braking, and only once in 5 years got below 5L/100km / over 47 mpg. People are easily misled by the fact that their engine doesn't use gas when decelerating and letting of the gas pedal. They don't realize they are also going to stop far, far sooner

You have to be on the gas longer to reach a coast point too. You lose on both ends when you coast in gear.

 RiceCake 08-08-2012 01:26 AM

Most people start coasting before coming to a stop, in that situation in gear will only be beneficial (engine doesn't need fuel to idle, free braking from the engine, etc).

Hypermiling is an entirely different ballpark so complaining that one is better then the other is apples and oranges. It also matters between cars and a lot of things.

Everyone argueing its "obvious" what the solution is is being shortsighted and ignorant. Test it and post your test conditions, your car, and your results.

 wdb 08-08-2012 05:34 AM

Quote:
 Originally Posted by Old Mechanic (Post 320409) Coasting in gear is not really coasting, since there is engine braking involved unless clutch disengaged in a manual).
Is that still as true as it used to be, this far along the trajectory of Moore's Law? My wife's 2011 CRV (they only sell them as automatics) rolls out pretty far in gear, when I'm moving at speed on a flat road; it doesn't quite feel like it's in neutral, but there is minimal engine braking taking place. When going downhilll however, I can really feel the engine braking the vehicle. I attribute the difference to Honda's transmission management logic.

 reb01501 08-08-2012 09:45 AM

Quote:
 Originally Posted by wdb (Post 320591) Is that still as true as it used to be, this far along the trajectory of Moore's Law? My wife's 2011 CRV (they only sell them as automatics) rolls out pretty far in gear, when I'm moving at speed on a flat road; it doesn't quite feel like it's in neutral, but there is minimal engine braking taking place. When going downhilll however, I can really feel the engine braking the vehicle. I attribute the difference to Honda's transmission management logic.
I don't understand the point you're trying to make. You start by questioning whether engine braking is occurring (and why is Moore's law relevant?), and then go on to describe you actually do observe it occurring. Seems to me you've answered your own question.

 roosterk0031 08-08-2012 10:52 AM

Automatics tranny engine braking depends on the car. Neither of my GM auto coast hardly any different OD or N. If engine braking is wanted need to put them in D or 2. Stratus engine brakes and downshift and brakes even more, so it needs to be shifted to N to coast well.

I wonder if the CRV is it keeping it's torque convertor locked going down hill to avoid speeding up. But unlocks it when coasting as long as not gaining speed.

The stratus if cruise is set and go down a hill large enough for it to pickup speed, will disconnect the tranny, the engine revs to 3,000 rpm, waiting for the car to get back down to the set point where the tranny will reengage, on hills that I know it will do it I will cancel cruise, shift to N, and then reengage tranny & cruise once the coast if over.

 UltArc 08-08-2012 11:19 AM

Quote:
 Originally Posted by jsatter (Post 320561) You have to be on the gas longer to reach a coast point too. You lose on both ends when you coast in gear.
I disagree with this. But only at certain speeds. When I go to the cemetary near me, I park on top of a hill. The scAngauge resets, and I roll forward and bump start into first, go to second and third, going back up the hill on the other side, she needs second, but by time I am out I get to almost 30 mph, 33-35 mpg, and use zero throttle pedal. Now I don't kow how much fuel I use, but it must be minimal, I put it into gear at 4xx rpm range and she wants to climb back up.

So for me, in THIS case, in gear coasting is very beneficial. Or maybe it's not technically coasting since I start at a stop. If this is the highway we are talking, then hands down, I totally agree. But unless I am 30+ mph, going uphill, or can see my stop point, I (think) I am better off in gear.

But those are strict standards. So I can happily agree usually out of gear coasting is more efficient than in gear.

 ecomodded 08-08-2012 11:51 AM

Coast down a secluded hill from a set starting point, till you reach a stop,mark the spot. Repeat it in neutral , repeat it all again. Then crunch the numbers.

 Fat Charlie 08-08-2012 12:04 PM

UltArc- Sorry, I wasn't trying to get in your face too much. But I was really hoping that you were joking in your hope that ksa8907 was joking.

To the challenge of "Find me some numbers," The only real answer here is "I can't." It can't realistically be tested for. The sheer obvoiusness of riding the brakes being bad for FE really turns the question around: Don't ask me to explain the benefits of coasting in neutral, explain to me why you think staying in gear is better than popping it into neutral.

 RiceCake 08-08-2012 04:05 PM

Quote:
 Originally Posted by ecomodded (Post 320638) Coast down a secluded hill from a set starting point, till you reach a stop,mark the spot. Repeat it in neutral , repeat it all again. Then crunch the numbers.
In neutral is essentially guaranteed to coast further but proves nothing. The real test would be to see if the extra fuel needed to idle the engine while in neutral is /less/ then the fuel saved by driving the engine with the transmission in gear.

 user removed 08-08-2012 04:18 PM

Coast in neutral, at 70 MPH, down a 5% grade on the Interstate (in neutral which is the DEFINITION of coasting) with a tractor trailer on yer butt, then shove it in gear and see what happens.

regards
Mech

 UltArc 08-08-2012 05:28 PM

Quote:
 Originally Posted by ecomodded (Post 320638) Coast down a secluded hill from a set starting point, till you reach a stop,mark the spot. Repeat it in neutral , repeat it all again. Then crunch the numbers.
I hope I remember to test this next week! I am rather curious about this now. At first, for MY situation, I thought it almost for sure, but really breaking it down, I don't know. I normally bump start my engine, next time I won't, so it stays fair. I think in neutral there will be a lot more variables in play. In gear it is smoother, but hopefully we'll see soon.

Quote:
 Originally Posted by Fat Charlie (Post 320643) UltArc- Sorry, I wasn't trying to get in your face too much. But I was really hoping that you were joking in your hope that ksa8907 was joking. To the challenge of "Find me some numbers," The only real answer here is "I can't." It can't realistically be tested for. The sheer obvoiusness of riding the brakes being bad for FE really turns the question around: Don't ask me to explain the benefits of coasting in neutral, explain to me why you think staying in gear is better than popping it into neutral.
Honestly, I did take it as hostile, but being it all in text, and this being a friendly forum, I figured I just took it the wrong way. And maybe my comment seemed hostile, too, so I try not to take any offense from anything. No harm, no foul.

I see your point in that, but some people are not very open minded :/ They get an idea, and that is just so until proven otherwise. My family is very much stubborn in this way. For years I have been telling them to unplug chargers and tvs and whatever they don't use, as it drains power. They finally did it to shut me up, and now they must constantly unplug everything that is not theirs, because of the energy they save.

Even on here, I remember someone switched to a funnel air intake rather than stock on a Focus I believe, and did not do very scientific reporting. The response was not very friendly lol. I know now to watch my exhuast when I talk about FE numbers lol.

 reb01501 08-08-2012 06:29 PM

Quote:
 Originally Posted by RiceCake (Post 320681) In neutral is essentially guaranteed to coast further but proves nothing. The real test would be to see if the extra fuel needed to idle the engine while in neutral is /less/ then the fuel saved by driving the engine with the transmission in gear.
That's why I wanted a net usage measure, not a measure of usage during the coast.

 reb01501 08-08-2012 06:34 PM

Quote:
 Originally Posted by ecomodded (Post 320638) Coast down a secluded hill from a set starting point, till you reach a stop,mark the spot. Repeat it in neutral , repeat it all again. Then crunch the numbers.
That won't prove anything. That's essentially what the writer of the PM article that started this discussion did. What's needed is a net usage result as gealii posted. What we're trying to prove is that the gas lost while coasting in neutral and regaining the momentum lost during the coast is less than the gas that would be lost trying to regain the momentum lost after coating in gear.

There is no question that coasting in gear loses less gas DURING the coast than coasting in neutral ...

 ecomodded 08-08-2012 06:51 PM

Quote:
 Originally Posted by RiceCake (Post 320681) In neutral is essentially guaranteed to coast further but proves nothing. The real test would be to see if the extra fuel needed to idle the engine while in neutral is /less/ then the fuel saved by driving the engine with the transmission in gear.
It will actually prove more than you think, i'll elaborate.

When in gear the motor / transmission become a brake if gas is not applied. Driving around with slight engine braking will lower your coast speed and length, meaning you will need to apply the throttle sooner then if you had it in neutral.

I have a sneaking suspicion that a car while in neutral and idling at 800 rpm is going to use less gas then a car engine braking at 3000 or more rpm.

these are the reasons i coast in neutral.

 reb01501 08-08-2012 07:04 PM

Quote:
 Originally Posted by ecomodded (Post 320711) It will actually prove more than you think, i'll elaborate. When in gear the motor / transmission become a brake if gas is not applied. Driving around with slight engine braking will lower your coast speed and length, meaning you will need to apply the throttle sooner then if you had it in neutral.
Well, duh! This is the point has been raised dozens of times in all these threads, without anyone documenting the savings - until gealii's post.
Quote:
 I have a sneaking suspicion that a car while in neutral and idling at 800 rpm is going to use less gas then a car engine braking at 3000 or more rpm.
Jeez! I thought we had established that with DFCO in effect, the engine is using ZERO gas, regardless of RPM, during in-gear coasting. It's the WHEELS making the engine's crankshaft spin at 3000 rpm, not the ignition of gas in the cylinders! The PM article does a good job of documenting this, all too well, unfortunately.

 ecomodded 08-08-2012 07:22 PM

Quote:
 Originally Posted by UltArc (Post 320693) I hope I remember to test this next week! I am rather curious about this now. At first, for MY situation, I thought it almost for sure, but really breaking it down, I don't know. I normally bump start my engine, next time I won't, so it stays fair. I think in neutral there will be a lot more variables in play. In gear it is smoother, but hopefully we'll see soon.
The differences in coast lengths could be computed as engine braking losses, i suspect that coast lengths will be shortened by 30% if so then the gas will need to applied 30% sooner?? which would need to be measured and calculated.
I have my preconceptions so a test would be cool.
I think people could expect at least a 10% improvement by using neutral over Drive,
i will add i use neutral constantly in my driving.

 Whitey 08-08-2012 08:57 PM

Hi, folks!

For what it's worth...

I've been driving my Honda for a bit more than a year now. A while back, I started using neutral for coasting while driving in town. Three tanks ago, I decided to start using neutral for coasting downhill on the highway.

I cruise at 57 MPH, and usually I'm able to maintain that speed during my neutral coasts. I had been unable to maintain that speed when "coasting" in gear and used a lot more gas. The evidence is in my fuel log (click the Honda badge in my signature). My last three tanks have been my three best tanks, averaging 47.01 MPG. The three tanks prior to those averaged 42.78 MPG.

-Doug "Whitey" Jackson

 roosterk0031 08-08-2012 09:08 PM

On the way home tonight with Cobalt I did a quick ABA, long gentle hill, about 120 mpg 5the gear 55 mph, was slowing 1 mph per 2 seconds or so, so best guess within 10 mpg. Accellerated back to 55 mph, neutral coast 170 mpg, just steep enough to maintain speed so a pretty good number. Back to 5th gear 55 mph again pretty much 120 mpg. I haven't calibrated my SGII for this car yet, still on first tank, but comparing factory gauge to SGII it's in the ball park.

So roughly (160/120) 1/3 less fuel is used coasting with my car, plus it holds speed longer so coast could be started sooner so even more is really saved.

DFCO doesn't really seem effective with the little bit I've played with it in the Cobalt. Even with aggressive down shifting for braking, fuel consumption actually goes up initially, only after atleast 2-3 seconds often 5 or more if it does at all will it go into DFCO. With my Cobalt I need to resist using engine braking as it's wasteful.

 ecomodded 08-09-2012 02:15 AM

Quote:
 Originally Posted by reb01501 (Post 320715) Well, duh! This is the point has been raised dozens of times in all these threads, without anyone documenting the savings - until gealii's post. Jeez! I thought we had established that with DFCO in effect, the engine is using ZERO gas, regardless of RPM, during in-gear coasting. It's the WHEELS making the engine's crankshaft spin at 3000 rpm, not the ignition of gas in the cylinders! The PM article does a good job of documenting this, all too well, unfortunately.
You have established absolutely nothing, besides being ill informed and slightly obnoxious,That i got.
Tell me how you suggest one measure for results?

 ecomodded 08-09-2012 02:37 AM

Quote:
 Originally Posted by Whitey (Post 320738) Hi, folks! For what it's worth... I've been driving my Honda for a bit more than a year now. A while back, I started using neutral for coasting while driving in town. Three tanks ago, I decided to start using neutral for coasting downhill on the highway. I cruise at 57 MPH, and usually I'm able to maintain that speed during my neutral coasts. I had been unable to maintain that speed when "coasting" in gear and used a lot more gas. The evidence is in my fuel log (click the Honda badge in my signature). My last three tanks have been my three best tanks, averaging 47.01 MPG. The three tanks prior to those averaged 42.78 MPG. -Doug "Whitey" Jackson
That is a 10% improvement, Sweet !

 ecomodded 08-09-2012 02:40 AM

Quote:
 Originally Posted by roosterk0031 (Post 320740) On the way home tonight with Cobalt I did a quick ABA, long gentle hill, about 120 mpg 5the gear 55 mph, was slowing 1 mph per 2 seconds or so, so best guess within 10 mpg. Accellerated back to 55 mph, neutral coast 170 mpg, just steep enough to maintain speed so a pretty good number. Back to 5th gear 55 mph again pretty much 120 mpg. I haven't calibrated my SGII for this car yet, still on first tank, but comparing factory gauge to SGII it's in the ball park. So roughly (160/120) 1/3 less fuel is used coasting with my car, plus it holds speed longer so coast could be started sooner so even more is really saved. DFCO doesn't really seem effective with the little bit I've played with it in the Cobalt. Even with aggressive down shifting for braking, fuel consumption actually goes up initially, only after atleast 2-3 seconds often 5 or more if it does at all will it go into DFCO. With my Cobalt I need to resist using engine braking as it's wasteful.
The engine is indeed running when coasting in gear, OBVIOUSLY, as the car is already started!!!! some people have neglected to notice that, somehow,right reb0151

Those were fast tests from you two guys, they confirm what i posted, thanks for the favorable results :-)

 ecomodded 08-09-2012 03:30 AM

here is that botched article from PM online.
All the same tomorrow i am going to test my car by coasting with the ignition off in gear then switch on the ignition and listen/feel for any change,like wise by testing with the ignition on in gear then switching it off.
Bust this myth, one way or the other.
I think the "Scangauge" reads injector pulse durations and has a function to read them,it would be useful to bust this Myth.
if anyone has one, take some readings while coasting in gear, engine on, its somewhere in the Scangauges menu.
No need to test with engine off as it will most definitely not be injecting any fuel.

the Article, finally !

Tactic No. 6: When Coasting Downhill, Leave the Car in Gear

There are those who refuse to be shaken from the practice of coasting downhill in neutral to save gas. This is a bad idea no matter how you look at it. Let's set aside fuel economy for a moment. Coasting downhill in neutral is illegal in most states. And it's dangerous in all states. In neutral, you have no way to accelerate to avoid a hazard, and if the engine stalls, you have no power steering or vacuum boost for the brakes. If the hill is steep enough to call for hitting the brakes to keep you from gaining speed, they're more likely to overheat—and overheated brakes lose effectiveness until they cool off. They'll probably do that right around the time the police show up to take the accident report.

Here's the surprise: There's no tradeoff between safety and fuel economy in this case. Leaving the car in gear while coasting downhill actually is more efficient. Why?

Most fuel-injected engines today use computer-controlled Deceleration Fuel Cut Off: When you lift your foot from the gas while leaving the car in gear, injectors shut off automatically, and the car's rotating tires—which are connected to the engine via the transmission—keep the engine turning and the accessories running. So, the engine consumes no fuel at all while the vehicle is coasting downhill.

In contrast, the fuel-consumption rate for an engine idling in neutral falls between 0.2 and 0.4 gallons per hour (gph). Splitting the difference and using 0.3 gph for our example, idling in neutral down a ½-mile-long hill consumes fuel for 30 seconds, for a total of about 0.32 ounces of gas. Popping the car into neutral actually wastes gas.

This may seem counterintuitive, but that's what data are for—replacing good guesses with solid facts. Watch the data, and over time the savings will take care of itself.

Read more: Driving Tips to Save Gas - Memorial Day Weekend - Popular Mechanics

Driving Tips to Save Gas - Memorial Day Weekend - Popular Mechanics

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