Popular Mechanics Article discusses use of Neutral Gear
 

Coasting in Neutral Does Not Save Gas: Claim Check

Has anyone done any tests similar to this? I'm sure there are some very strong opinions about this and just wondering what the proper procedure to save the most $$ would be?

I think they are dead on in the article. It makes complete sense, which is why I always laugh when people tell me they go into neutral to save fuel.

I drive a MT car and I always gear down as I decelerate. It saves on your brakes as well as saving fuel. Many argue that it shortens clutch life, but on my current car I am at or above the average life span many others report.

Unfortunately I can't compare my numbers accurately on here, but on a forum devoted to car & engine type I am on the high end of MPG when comparing numbers.

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?

I do know by using the guage on my car (which I've found to be fairly accurate comparing my calculated numbers from here) I use much less fuel coasting down a hill in gear, than when my car is idling.

There's no need to downshift unless you'll be coasting down a steep hill and you need to keep your speed down (as you do use gas to rev-match, but you use zero gas coasting in any gear as long as you're above 1500rpm or so).

But there's no doubt that coasting in gear takes zero gas and coasting in neutral takes a little bit of gas. If you're going to be coasting down a slight hill for a long ways and leaving it in 5th gear will cause you to slow down, that is almost the only situation where you'd want to be in neutral, but those hills are few and far between.

Otherwise, the vast majority of drivers are usually coasting towards a stop light, traffic, or a turn, and need to reduce speed anyways, so leaving it in gear both slows you down and uses no fuel. But the amount of fuel used while idling (especially in new cars) is so small that it won't make a huge difference either way.

Living in Wales, i have experimented with this. Obviously being in top gear and NOT accelerating uses no fuel as the engine management cuts fuel on the over ride. But it uses fuel to idle (e.g. if you're in neutral) I wouldn't recommend putting a car in Neutral if it has an automatic gearbox though (urgh!)

However there is a hill near me that you can travel 6 miles in neutral. You cannot do this in top gear cruising as the revs and resistance in the gearbox slow you down! So you can either travel further but maybe use more fuel, or use less fuel but not go as far...

So, i think i need to do some more experiments!

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.)

This maybe one of the reasons automatic transmissions are now getting better mileage than manual transmissions. I'm sure there are other additional reasons, but it is probably a contributing factor.

Your MPG when coasting in neutral is still very good (30-80mpg depending on speed) but the fact that you could be getting infinity mpg (using no fuel at all while still traveling) means you are wasting a little fuel.

Modern auto transmissions are starting to get better MPG simply because they have more gears and at least one ultra-long cruising gear. Since they shift quickly and automatically, you can maintain speed at very low rpm with a small motor at high speed, and the driver won't care because it will shift to a lower gear as soon as they want more power. Drivers with manual transmissions don't tolerate as long of 5th/6th gears simply because they don't like downshifting whenever they need to gain speed. Note that the autos still all have lower city mpg ratings than their manual-trans counterparts. The OEMs figured this out a long time ago, but that was when extra gears in transmissions were more expensive than CAFE penalties.

Neutral and engine off saves fuel. Not saying its a good idea but if you are that concerned about the fractions of gallons and dont mind suddenly not having power steering (or brakes) then give it a go. Although it is going to take so much muscle at that point you might as well be riding a bike. Or pushing your car.

Like everyone has agreed staying in gear with a "neutral" throttle position, not engine braking but close to it, provides maximum sensible fuel conservation.

Locking torque converters found in modern automatic transmissions also help.

Oh and can i just remind those of you trying to save fuel, that we in the UK are paying around $8 a US gallon for fuel!

Tip: Want to save money on fuel? Move to the US! (or better still, the Oil rich Eastern countries where it's just a few pence per litre)

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.

>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.

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.

>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 :-)

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

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

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.

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.

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.

@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).

tommyz my fiat 500 says 99.9 when in neutral, and less when using engine braking.

This subject is not clean cut, the scenarios would determine the method to use. Trying to say one way or the other is best is like saying Marmite is 'orrid/lurvely.

I personaly get more mpg from coasting, especially on the flat wher I can coast for 1/4 mile in some areas, before having to decide to brake or slip it back into to get around the roound about.

MMUK has makes a valid point about steep hils though.

Don't forget that turning the engine, either way requires energy.

Recently installed a an engine analyzer on my 335d. It is the ECO Route by Garmin that connects to the OBCDII port and links via bluetooth to the Garmin so you can see mutiple engine parameters.

Some of those are engine load and air mass flow. Stopped at a red light in Drive I get a slightly higher Engine Load and Air Mass Flow than when I tip it into Neutral. Same goes for the A/C being turned on or off.

So, definitely sitting in Neutral will save you some fuel, but is it worth it? Depends how much time you sit waiting at stop lights.

".......cars don't handle well in neutral during sharp cornering maneuvers when the engine isn't connected to the drivetrain."

Now this had me howling with laughter. Take a FWD vehicle, go into a slippery corner and lift off the gas suddenly. You'll go into trailing-throttle understeer very quickly and slide straight ahead. Now try it but dip the clutch (neutral) instead and the car will track right around the corner. Conversely, lift quicly in a RWD in a sharp slippery corner and the car will likely swap ends as the back wheels skid out (trailing-throttle oversteer). The author of the article obviously has no knowledge of vehicle dynamics.

It is too bad PM published such a badly written article.

In my experience coasting in N does save gas, unless you need to brake. Then you should use the fuel cutout and leave the car in gear.

PM must be worried about people coasting in N for some reason. Its not like they don't review certain cars because they can break any speed limit in 6 seconds or less...