It was always clear we were talking about F.I (no carbed Honda Fits) But I do know what you are talking about,we used to do this for laughs in a certain tunnel in Seattle.
I did this many times during high school, and yes, I destroyed about three mufflers.
It always amazed me how loud a bang it made and the size of the flame it threw out the exhaust.
So much about this thread is…boooogus!
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Legality: Depends upon where you live. In PA, there is no specific law against coasting, be it engine on or engine off.
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Safety: My understanding is that the reason for laws against coasting is that trucks (and probably cars from back in the “old-timey days”) needed engine braking as a redundant safety setup for underpowered brakes. Now–especially on an interstate, where maximum gradients are quite low–no modern car will be close to maxing out brake energy. The practice might be slightly more dangerous than the status quo, but so is driving in the rain and/or nighttime.
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Efficiency: You will absolutely save gas “coasting the downhills”; you’ll just save more engine-off. In fact, you’ll probably do better in hilly terrain with this method than you’d do on the flat: by (of necessity) using the engine heavily on uphills and not at all on downhills, you’re mimicking the “pulse-and-glide” techniques used in MPG contests to maximize mileage.
I find that I net a 25% fuel savings in hilly terrain with this technique–2c/mi at current fuel prices. I strongly suspect this will cover any increased wear issues, though I have a “before” and “after” oil sample that I’ll send off to analysis to quantify any increased wear.
The big advantages I can see to “free wheeling” as we ex SAAB ice racers used it, is to keep a two stroke from loosing sufficient lubrication w/o gas oil mixture at higher speeds, and enhancing steering control in icy conditions where a non braked or accelerating wheel maximizes steering traction control at lower speeds. That’s why ABS works. Otherwise, I vote a big NO to neutral with the rest do to other safety considerations.
You want a big lesson on safety in leaving a vehicle in gear where the results are greatly magnified and not quite realistic to cars but relevant; slip a 4000+ lb tractor into neutral going down hill letting your speed build up and try to control your descent with just the brakes. If you survive, you may think twice about your suggestion.
I would have no problem with writing and enforcing laws outawing coasting for vehicles requiring a CDL to operate.
Can you really not picture any emergency situations where you’d need to accelerate immediately? What if you braked hard to avoid something that darted across the road and then you realized you needed to floor it because of traffic coming up behind you quickly? What if you swerved into the oncoming lane to avoid something and then you needed to get back into your lane immediately? What if you were startled by the horn of a truck that lost its brakes about to hit you from behind? What if your car started to oversteer, which is something that a skilled driver might accelerate to recover from?
Meanjoe75fan wrote:
So much about this thread is…boooogus!
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Legality: Depends upon where you live. In PA, there is no specific law against coasting, be it engine on or engine off.
-
Safety: My understanding is that the reason for laws against coasting is that trucks (and probably cars from back in the “old-timey days”) needed engine braking as a redundant safety setup for underpowered brakes. Now–especially on an interstate, where maximum gradients are quite low–no modern car will be close to maxing out brake energy. The practice might be slightly more dangerous than the status quo, but so is driving in the rain and/or nighttime.
-
Efficiency: You will absolutely save gas “coasting the downhills”; you’ll just save more engine-off. In fact, you’ll probably do better in hilly terrain with this method than you’d do on the flat: by (of necessity) using the engine heavily on uphills and not at all on downhills, you’re mimicking the “pulse-and-glide” techniques used in MPG contests to maximize mileage.
I find that I net a 25% fuel savings in hilly terrain with this technique–2c/mi at current fuel prices. I strongly suspect this will cover any increased wear issues, though I have a “before” and “after” oil sample that I’ll send off to analysis to quantify any increased wear.
Wow!
Wow…how, exactly?
- and 3. are verifiable statments of fact. 2. is debatable, but I have not seen any reliable quantitative evidence stating coasting is “extremely dangerous,” which seems to be mentioned in the “everybody knows” sense.
(Obviously I can envision a scenario in which being in gear would save me, just as I can envision a scenario where I’d be better off unbelted. Without quantitative risk assesment, though, it’s impossible to say how much additional risk–if any–is entailed by this maneuver.)
This reminds me a lot of the “is the safety benefit leaving it in gear (clutch in) at a red light worth the wear-and-tear on the throwout bearing?” There was a similar point raised about how one might occasionally avoid an accident this way, but the net safety benefit would be unlikely to pay for the throwout bearing wear. I posit that the safety benefit of not coasting is similarly sized.
meanjoe75fan: Did you miss all the postings in this thread, not to mention all the writings in car magazines by engineering editors that modern cars shut the fuel OFF when coasting in gear? Engineers sometimes call this “over-run mode”. You can not use less than zero fuel by turning the engine off.
doubleclutch:
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Just because a vehicle has “fuel cut-off” capability does NOT mean that it’s actually USING it. I bought a guage that accesses my OBDII data, giving me (among other things) fuel economy. It only goes to “9999 MPG” (i.e. fuel off) in select instances: it’s good about cutting off fuel when I take the exit ramp off of a highway, but that’s about it. To get it to cut-off on a downhill, I have to downshift to the point that my engine sounds uncomfortably like a Pratt+Whitney turbine!
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Fuel cut-off is NOT free. Instead of using gasoline to turn my enigne at 900 RPM (idling), I’d need to use kinetic energy (forward motion) to turn the engine at 2500 RPM (highway cruise). Now, the process of conversion of gasoline to motion is less efficient, but the pumping losses of 2500RPM are so much higher that one can ultimately get better mileage idling (experimentally observed).
Granted, IF one is descending a hill sufficiently steep that one can maintain forward speed in gear AND one knows that the vehicle is in fuel cut-off mode, THEN yes, neutral is pointless. I was only talking about hills insufficiently steep to maintain speed in gear…which is a big reason why I don’t think the “Danger, Will Robinson!” safety comments (regarding brakes) wash.
(Since my car rarely cuts off fuel on truly steep hills, I go to “acc” on the ignition: still have PS+PB in that case, plus I know “for sure” I’m not burning fuel!)
I think for most modern cars it’s pretty rare that there’s a hill that’s just the right slope that you can just coast in neutral with no brakes, but would need to use some gas to keep it rolling in overdrive.
Back in the old days you would have been right because of carburetors, which supplied fuel regardless, and because older cars were geared so much lower. On my old '76 Chevy truck with a 3-speed, you definitely do have to use gas to keep the thing going downhill at highway speed in third gear, so you can definitely see where the temptation was. Of course, older cars also had crummy brakes that were prone to brake fade and out and out brake failure due to excessive heat build up, which is why pretty much every state that has substantial mountain passes has laws against coasting.
I agree that it’s probably not much of an issue on a modern car, but I also think beause of the prevalance of overdrives and the fuel-shutoff feature, it would require a very improbable combination of car and hill to have coasting save you any gas at all.
Meanjoe75fan wrote:
Wow…how, exactly?
What I found most troubling about your reply is how you totally discount all the input the numerous contributors provided on this subject - and you did it with a tone that your view was the only correct view.
Unfortunately, doing that removes any credibility from anything you wrote.
meanjoe75fan, I keep hearing about how sitting with the clutch pressed will wear out the throwout bearing, but in all of history, has anyone ever seen an otherwise healthy clutch with a worn out throwout bearing? It seems to me that the throwout bearing is designed to be used that way.
Deceleration Fuel Cut-Off (DCFO) – only on “modern” cars, that is to say, engines with computer-controlled fuel injection (available industry-wide in the U.S. 1995 and after).
I have seen plenty of VW’s with perfectly good clutch discs and pressure plates but a host of other problems with the clutch (almost to numerous to take the time to write).
Everyone knows how I feel about VW’s,overated death traps with problems that never in a million years would be found on other type cars.
Gotta love thoses problems,kept me in beans for many years.
Coasting downhill in neutral is illegal in my state, but like you said, how can they catch you 
I prefer puting it in neutral and leaving the clutch out. This will save you 2 miles worth of wear and tear on the throw out bearing and the clutch pressure plate springs.
Contrary to what was said above, I can save fuel if I coast in neutral. If you left it in gear and just backed off the throttle, your engine rpm would be comensurate with the vehicle speed (well above idle) and it would use MORE fuel.
I don’t know what your vehicles final drive ratio is, but mine would save approx. 65% to 70% in fuel useage @ 60mph due to the difference in engine rpm @ 60 mph versus 800 rpm @ idle. Also be mindfull of Whiteys warning above re: loseing power steering and power brakes… IF THE ENGINE DIES, you WILL lose p.s. & p.b. This is the only danger I see
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DISREGARD this post if you are driveing an f. i. vehicle with the computer controlled fuel shut off system. I wasn’t aware that “fuel shut off systems” existed untill reading the posts here.
The idea of putting a transmission, auto or manual in neutral coasting is akin to putting an auto in neutral at every stop sign, a practice we’ve likewise discussed. Is there some benefit to doing either ? Are there isolated instances where it might be advisable ? Perhaps, but the realizing of such benefit usually comes at a cost, somewhere else. I don’t believe the automotive engineers who design the cars ever intended that such practice would be common beyond what’s stipulated in the owners manual. They probably laugh themselves silly at the lengths some people will go to save a little gas at the expense of time, safety and wear and tear on more expensive mechanics while driving their creations.
Of the many engineers I personally know as friends or have worked with, none engage in this practice or have even considered it while driving their BMWs and the like. I never ran the practice by them as I never wanted to get that; “Are you foolish or what?” look from them.
Read again-- on fuel injected cars the computer turns off the fuel entirely because the downhill momentum of the car keeps the engine rotating. Even though the engine is turning at 2000 RPM or whatever, it’s using ZERO fuel and you are getting infinity miles per gallon.
If you shift into neutral, the engine is no longer connected to the drive wheels and thus cannot keep the engine rotating. Therefore, the computer has to add fuel. Granted, you’re getting some incredibly large MPG number, but it’s not bigger than infinity!
As I mentioned above, the ONLY way you’ll save fuel coasting is in a situation in which you need to have your foot on the gas to maintain speed while in gear, but you’re going JUST the right speed in neutral. I suspect that given the gearing of modern cars, this probably practically never actually happens.
Re: The idea of putting a transmission, auto or manual in neutral coasting is akin to putting an auto in neutral at every stop sign, a practice we’ve likewise discussed. Is there some benefit to doing either ?
I never saw the discussion on that topic so I don’t know what conclusions were reached regarding the benefits. If it’s going to be a long stop light (more than a minute) & I’m driveing a manual trans I usually put it in neutral and let out the clutch to save the wear and tear on the t. o. bearing and the pressure plate springs. I figure several thousand minutes of wear saved at stop lights is just added to the life of these 2 items at the other end, but no fuel saveings because you are at idle rpm either way…
With an auto trans I really don’t know. Under the same circumstances, I think it may be a wash, or uncalcuable (at least by me) if you kick it into neutral. You would be saveing wear and tear on all the slipping clutches and associated pieces and parts if left in gear, but if you put it in neutral the idle will increase 300 or 400 rpm so will use more fuel. I don’t have the knowledge of the math skills to equate worn out trans part to fuel used in this situation. Do you have any thoughts on this?
BTW I have seen that “look” you are talking about from the district rep several years ago when I thought it was alright to ask questions lol
This “shut off system” is interesting and you have explained it well. Is it used on ALL new cars now? Import and domestic? I knew about the systems that would shut down selected cylinders after getting up to speed and the computer decided it could maintain vehicle speed on 1/2 the cylinders, but this is new to me. Thanks for the explanation.