Popular Mechanics Article discusses use of Neutral Gear

[CyberGuy]

I tried this experiment just last year from May thru July 2010. I have a 1999 Ford Ranger with manual transmission. I would coast to a stop in neutral and on most hills shift into neutral. At the end of July I returned to driving the normal way. I can say without a doubt that shifting into neutral and coasting did increase my gas mileage. You can see the results on my fuelly page.

[MMUK]

>And honestly, if shifting to neutral saved that much fuel and

>caused no safety risk, don't you think auto manufacturers

>would make the car do so automatically?

errm... but that's exactly what some cars do now, in order to save fuel.

The article's author didn't analyse the situation fully. You can save fuel in neutral, but only in certain scenarios, and it's not a great deal. In other scenarios you're better off in gear - it just depends on whether you need to slow down or not.

[MMUK]

In fact, I looked at the article again, and I see that he is assuming that idle takes 1 gallon per hour. For diesels, the true figure is about 20% of that - roughly 0.2 gallons per hour.

So by the author's own (deeply questionable) logic, his 30 mph example corresponds to 150 mpg. (Now, the reality is that it makes no sense to go into idle at 30mph, because the RPM in 5th will be around idle speed in any case, so in a diesel with plenty of low-end grunt, you're probably best off in gear). Going into neutral only makes sense when a) your RPM will be significantly lower as a result (since it is the engine resistance which will be making a difference to fuel consumption), and b) you don't need to slow down.

If you DO need to slow down, you should ALWAYS use engine braking & be in gear - the fuel consumption drops to zero because the wheels are driving the engine, rather than the other way around.

Basically he has deliberately picked a high idle consumption and a low speed in order to bend the the facts to suit his argument.

[MMUK]

>Keep the car in gear. Just put your display on current (instant) MPG

>and you will see that if you go into neutral the display will go blank,

>meaning you aren't getting any miles to the gallon.

> But, if you stay in gear you will see the MPG shoot up to 99.9 MPG

>(their way of saying the fuel is shut off.)

A couple of misunderstandings here.

You are correct when you say that if you stay in gear, the fuel is shut off (hence you are doing infinite MPG).

However...

When you go into neutral, you are not getting zero miles per gallon. That just doesn't make sense if you are moving.

You *are* consuming fuel - at the rate needed to spin the engine at 850rpm (or whatever your vehicle's idle speed is). You may think that this means that your overall mpg will be less than if you were in gear. This again, is a misunderstanding - when you are going down the hill, you are using your kinetic energy to spin the engine at high RPM. So although you are not using any fuel going down the hill, in the long run you will need to use more fuel to accelerate back up to speed.

If the hill is steep enough that you start to go faster when you are in neutral, that signifies that you should be in gear, since burning off kinetic energy at that point is good :-)

[rrounds]

In my car(S2000) I put it in neutral all the time when going down hill. If I don't, I have to press on the gas pedal or the car will slow down but put it in neutral and the car will pick up speed(up to 83 on some hills). With a 4.10 rear end gear(stock) the car does slow down when you take your foot off of the gas pedal even on 8% grade hills.

ROD

[OggyGTA]

Christ! What speed are you coming down those hills at?

[i90east]

The weight of the vehicle is probably a significant factor here. I have a very small and lightweight car. When I coast in gear I lose a lot of speed because there's not much mass moving the car forward and countering the engine drag. I would have to pulse very often to get my speed back up with would nullify the benefit of coasting in gear. Without a doubt I get better MPG coasting in neutral as much as possible compared to always keeping the car in gear. I only coast in gear when I need to lose speed.

Perhaps with vehicles that are a few hundred pounds heavier than mine coasting in gear makes more sense.

The safety concerns in the article are laughable.

[kevm14]

Deceleration fuel cut off (or DFCO) has been around as long as fuel injection. It's nothing new. That said, it is obvious that while in DFCO the car is using no gas, assuming the injectors aren't leaking. In neutral, it uses about as much as it would at idle, in the case of the manual trans.

However, their generalization that "cars use no fuel when coasting in gear" is far too broad to make. It all depends on the PCM calibration. I know on my cars, DFCO isn't used below a certain speed, or below a certain rpm. Coasting in 6th gear at 45mph, for example, will not engage DFCO.

Also remember that even when using no fuel, a turning engine is still an air pump, and you are still trying to create a vacuum against a nearly closed throttle blade. This uses energy and will slow you down more than if you were in neutral, particularly if you need to downshift in order to select a gear that WILL engage DFCO.

So the short answer is:

If you can create conditions under which the PCM will engage DFCO mode AND the extra drag doesn't lead to an unsatisfactory speed, then use DFCO.

If your conditions don't enable DFCO OR the extra drag won't work in your situation, coast in neutral.

In my case, on my GM V8s, it is easy to tell if I am in DFCO. I also have scan tools for each car (and access to each computer's calibration should I decide to CHANGE the DFCO parameters), so I can double check that way.

[MatrixDom]

I've always geared down to decelerate instead of putting it in neutral. According to my ScanGuageII I get 0MPG (infinity) mileage when gearing down as opposed to coasting in neutral where the engine still uses gas.

I believe the injectors go in deceleration mode as soon as you let off the gas pedal.

[MMUK]

@Matrix: If you want to decelerate, yes, you should be in gear. But if you want to stay moving, neutral is better (unless you are going down a steep hill).