Manual transmission: coasting in gear v. in neutral

driving
gasoline
manual-transmissions
transmissions

#1

I have a 2001 Civic HX w/ manual transmission, but this question applies to all modern manual tranny cars: Which uses less fuel: coasting in neutral or coasting in gear? Esentially, coasting in neutral means idling. I read somewhere that with modern, fuel injected cars, when coasting in gear (so that there is no load), the computer shuts off the fuel flow to the injectors, so that coasting in gear actually uses less fuel than coasting in neutral. Is this accurate/true? Thanks.


#2

It depends on the situation. If you need to use the brakes anyway because you are arriving at a red light too hot to just coast up to it, keep it in gear so that the fuel injectors completely cut off. (not all cars do this)
If you can just coast up the the intersection in neutral, then the engine braking with the car in gear will require you to use some gas to make it to the intersection and there goes all the savings of the fuel cutoff.
Four cyclinder engines typically burn .2-.3GPH during idle with AC off, about 25% more with ac on and furthermore, the deceleration fuel cutoff point is usually higher with AC on.

The hypermiling experts mostly say that the energy you throw away by engine braking in gear more than offsets the fuel savings of DFCO, and these people measure their results.


#3

Depends on the car and the situation. Many modern cars turn off all fuel when the accelerator is not depressed and the engine is turning over faster than idle. Some don’t. Which will save more fuel will depend on how long how fast what hill etc. However the most important issue is safety so start there and only adjust to safe fuel if it does not compromise safety.

If you would otherwise use your brakes to slow down, then use the engine for braking, of not … It is up to you not much difference in most cases.


#4

Yes, safety first.

The Civic HX is not a hybrid and therefore does not have regenerative braking. I mention this only because some people assume “H” is for hybrid.

It does have the VTEC-E engine (“E” for economy – at least that designation is logical).

Occasionally I drive down long, continuous, mostly straight grades varying in length from 3 to 22 miles. In this car, on these particular grades, the factors of wind resistence, rolling resistence, vehicle mass, and degree of grade coincidentally allow a coasting-in-neutral velocity approximating the posted speed limit within 10mph for much of the distance. I had assumed, in that context, that coasting in neutral was saving fuel relative to coasting in gear, until I read about the fuel injection shutoff.

So anyway, thanks for the information.


#5

“… the energy you throw away by engine braking in gear more than offsets the fuel savings of DFCO …”

Forgive me, but I don’t understand that sentence. Are you saying that, in the context of approaching a complete stop (as opposed to a miles-long downhill grade), coasting in neutral for a slightly longer distance will use less fuel than coasting in gear (thus braking with the engine and engaging DFCO) for a shorter distance?

I would expect the difference to be small, but cumulatively, possibly noticeable and maybe even significant.


#6

If you approach an intersection with so much speed that you can coast up to it with the engine engaged in DFCO, then the uneeded fuel has already been burned. There’s nothing you can do about it now except hope the light turns green before you stop. Might as well leave it in gear.

On a shallow downgrade where your car just only maintains cruising speed in neutral, leaving it in gear means you have to give the engine enough gas to not engine brake the car. Let’s say 50 mph and lets say your car’s fifth gear has the engine spinning 2000 rpm at that speed. In order to not engine brake the car to a lower speed, you have to rev match the engine to the car with the throttle and you will be burning the same amount of fuel as it would at a 2000 rpm fast idle as opposed the the fuel it would burn at a 650 rpm idle in neutral. You have to close the throttle for DFCO to kick in and when you do that, you slow down real fast and have to re-accelerate to get back up to that 50 mph cruising speed and there goes all the fuel you saved in DFCO.


#7

I think that in the situation you describe, coasting in neutral would probably be more economical, because trying to coast in gear will slow you down, so you will have to push on the gas periodically to get your speed back up. Since that will occur at 2500 RPM vs 500 RPM, you are probably better off in neutral.

HOWEVER - coasting in neutral is illegal in the states where I am familiar with the traffic laws… not that there is any way you could ever get caught…


#8

The “H” in the Civic HX stands for high mileage. The Civic HX has been around since before hybrids were for sale to the public.

One more thing. If there is any fuel savings form coasting in gear, it isn’t enough to make downshifting your preferred method of slowing down. It is okay to use engine braking in your current gear, but downshifting through the gears will increase wear to your clutch.


#9

I have confirmed to my satisfaction that the answer to my original question is, Yes, modern computer-controlled fuel injection will stop injecting fuel while coasting in gear, as the wheels will turn the engine and all of the accessory belts. I am glad I asked, because this was news to me.

I presented this same question over at a Consumer Reports forum, and their staff expert said:

“We have fuel meters that are tapped into the lines to read actual fuel consumption. Coasting in gear uses no fuel to speak of. Coasting in neutral still requires fuel to keep the engine running. The most efficient way is to come to a stop in gear for as long as possible and coast down hill in gear. [emphasis mine]

Any time you are engine braking, you are using no fuel to speak of.

If you have a Consumer Reports membership, you can view that thread here: http://discussions.consumerreports.org/n/pfx/forum.aspx?msg=4234.1&nav=messages&webtag=cr-38aujohn
Post numbers 2 and 8 were the most informative.

By the way, no part of my consideration of this question involves turning off the engine at any time.

Thank you all . . .


#10

I don’t know if Honda’s “H” designation officially stands for high mileage or not, but it certainly could. I average 42mpg combined city/highway.


#11

The disk brakes on your wheels also use no fuel whatsoever when you apply them. Yet, one of the effective ways to increase your gas mileage is to drive to use your brakes as little as possible.
If the people at Consumer’s Report ever mangage to make a stock 5speed Yaris get 45+ mpg, I’ll gladly listen to what they say, until then, I’ll take the advice of the group of people who actually do get that kind of gas mileage.

However, if the situation is such that you have to use your brakes anyway, you should leave it in gear and cut off the fuel.
You can actually feel when the fuel comes back on as you coast down, there is a sudden decrease in engine braking when that happens. On my Yaris, this happens at about 17 mph in fifth gear, 24 mph if the AC is on. I bump it into neutral as soon as I feel the DFCO (deceleration fuel cut off) end.