Which is more fuel efficient?

[cozmik]

So I have a question, which is more fuel efficient, cruising to a stop with the car in gear or out. This is with a manual transmission. My friend is telling me it's better to keep it in gear, but part of me thinks he's wrong because of how many other things he's wrong about.

[93JC]

The difference would be pretty much negligible, I think...

[heelntoe]

i would say that staying in gear saves an ever so little amount of fuel. in neutral, the car is basically idling and still using gas, but in gear, when you're off the throttle, there is no fuel being burned.

[S204STi]

This is an ongoing topic of discussion.  My view:

In gear the inertia of the vehicle is keeping the engine running.  Many cars are smart enough to sense it and cut fuel altogether.  In neutral the engine has to feed fuel and air to itself to keep itself running.

[MX793]

Many cars shut down (or very nearly shut down) the fuel injection under engine braking, which uses less fuel than idling (as you would be if coasting with the tranny in neutral or clutch disengaged).

[JWC]

This is an ongoing topic of discussion.  My view:

In gear the inertia of the vehicle is keeping the engine running.  Many cars are smart enough to sense it and cut fuel altogether.  In neutral the engine has to feed fuel and air to itself to keep itself running.

Many cars shut down (or very nearly shut down) the fuel injection under engine braking, which uses less fuel than idling (as you would be if coasting with the tranny in neutral or clutch disengaged).

I thought both of those scenarios applied to auto trans more than manual trans.   Might be I'm thinking of the "old days" when we were taught to save gas by avoiding jackrabbit starts and stops.

But, there's something new to learn everyday and computers monitor everything.

[r0tor]

Many cars shut down (or very nearly shut down) the fuel injection under engine braking, which uses less fuel than idling (as you would be if coasting with the tranny in neutral or clutch disengaged).

+1

[ifcar]

It depends on the car and the situation.

At home, we have ScanGauge mpg monitors in a 2004 xB and 2005 Focus, both stickshift. In the Scion, the monitor shows fuel supply being cut off when you're in gear and not touching the gas. That's not the case in the Focus, however, which gets better mileage if you take it out of gear.

Another factor to consider is that you don't get engine braking in neutral, so you can coast farther out of gear than in.

[AutobahnSHO]

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[giant_mtb]

Why are spammers so illiterate?

[Byteme]

Many cars shut down (or very nearly shut down) the fuel injection under engine braking, which uses less fuel than idling (as you would be if coasting with the tranny in neutral or clutch disengaged).

Our Mazda 3 shows mileage of 99.9 MPG when I take my foot off of the gas and coast in gear.  Tonight I'll try a couple of slow downs coasting in neutral and see what the readout reports.

I think it still uses a tiny bit of fuel even when coasting in gear because as I get really slow the mileage readout changes from 99.9 to 67, then to 34.  Then I'm pushing in the clutch and braking.

[Byteme]

I thought both of those scenarios applied to auto trans more than manual trans.   Might be I'm thinking of the "old days" when we were taught to save gas by avoiding jackrabbit starts and stops.

But, there's something new to learn everyday and computers monitor everything.

According to Road and Track one should accelerate with large throttle openings and shift early to get into the higher gears at a lower speed.  The large throttle openings reduce pumping loses while the early shifting ensures you aren't burning a bunch of gas at high revs when the car could be perfectly happy at lower revs at the same speed.

[sportyaccordy]

According to Road and Track one should accelerate with large throttle openings and shift early to get into the higher gears at a lower speed.  The large throttle openings reduce pumping loses while the early shifting ensures you aren't burning a bunch of gas at high revs when the car could be perfectly happy at lower revs at the same speed.

What if you drive Honda

[NomisR]

According to Road and Track one should accelerate with large throttle openings and shift early to get into the higher gears at a lower speed.  The large throttle openings reduce pumping loses while the early shifting ensures you aren't burning a bunch of gas at high revs when the car could be perfectly happy at lower revs at the same speed.

So you take off at WOT and shift immediately to the highest gear? 

[MrH]

A couple of you are saying that it shuts off fuel injection completely.  To me, that doesn't sound right at all.  That means no combustion.  There's no way it's just pumping air at that point.  The exhaust note would change drastically.  Everything would change.  The car would slow considerably more when combustion isn't happening.

I can see it cutting fuel off a lot, along with air flow, but not completely.  That doesn't even make sense.

[S204STi]

A couple of you are saying that it shuts off fuel injection completely.  To me, that doesn't sound right at all.  That means no combustion.  There's no way it's just pumping air at that point.  The exhaust note would change drastically.  Everything would change.  The car would slow considerably more when combustion isn't happening.

I can see it cutting fuel off a lot, along with air flow, but not completely.  That doesn't even make sense.

Let me show you something...



This is a Buick LeSabre, 2005, 3.8L V6 engine.

As you can clearly see, it cuts off fuel on decel.

As for the drastic changes you imagine, well, have you heard and uncorked car decelerating for a turn?  It does in fact sound different.  You just can't hear it most of the time with your muffler installed.  Pumping air isn't a big deal for a car, anyway.  Displacement on demand cars do that in fact.

[Raza]

Well, you put the clutch in as you come to a stop anyway, so what's the difference? 

[S204STi]

Well, you put the clutch in as you come to a stop anyway, so what's the difference? 

I don't put the clutch in till I am nearly to a complete stop.

[Byteme]

What if you drive Honda

I'd be ashamed of myself.

[Byteme]

So you take off at WOT and shift immediately to the highest gear? 

I use a lot of throttle and shift at about 2000-2500 rpm  I usually skip 4th and go from third to fifth.

[Soup DeVille]

Let me show you something...



This is a Buick LeSabre, 2005, 3.8L V6 engine.

As you can clearly see, it cuts off fuel on decel.

As for the drastic changes you imagine, well, have you heard and uncorked car decelerating for a turn?  It does in fact sound different.  You just can't hear it most of the time with your muffler installed.  Pumping air isn't a big deal for a car, anyway.  Displacement on demand cars do that in fact.

thought DoD engines shut off the intake/exhaust valves, so that they don't actually pump any air while they're shut off.

[S204STi]

Maybe you're right!  I'm not entirely sure.

[MX793]

thought DoD engines shut off the intake/exhaust valves, so that they don't actually pump any air while they're shut off.

Unless they're direct injection, they'd have to.  Otherwise, fuel would still be going into the cylinder and if the ignition was shut off, you'd get a bunch of raw gasoline spewing out your tailpipe.  Not to mention the added drag of the piston pumping air (vs being an air spring).

[S204STi]

Unless they're direct injection, they'd have to.  Otherwise, fuel would still be going into the cylinder and if the ignition was shut off, you'd get a bunch of raw gasoline spewing out your tailpipe.  Not to mention the added drag of the piston pumping air (vs being an air spring).

Or they could just not pump fuel to the cylinder, but that would still lean out the a/f mix in the exhaust.

[Raza]

I don't put the clutch in till I am nearly to a complete stop.

Interesting.  I put the clutch in when I drop below a speed appropriate for that gear, in case emergency acceleration is needed, that way I can get into gear faster. 

[MX793]

Interesting.  I put the clutch in when I drop below a speed appropriate for that gear, in case emergency acceleration is needed, that way I can get into gear faster. 

I practically always keep the car in the most appropriate gear for the speed I'm travelling and keep the clutch engaged as much as possible.  That way, I don't even need to shift into gear and drop the clutch, I can just step on the gas if the situation calls for acceleration.

[MrH]

Or they could just not pump fuel to the cylinder, but that would still lean out the a/f mix in the exhaust.

You'd still have the added drag of pushing the air around.  Better to just shut the valves and let the pistons compress the same air over and over.

[565]

For what it's worth, on the Z06 the DIC tells me I get much better mileage when I coast with the gear out and at idle compared to being in gear with the RPM's up.  I did this experiment sometime during the summer.  With the car in gear and going down a hill the DIC told me I was doing about 60ish MPG.  In neutral and just coast the MPG will jump to 99.

I'm not so sure about the fuel being instantly cut off when you lift off the gas in the Z06.  I mean coming down from high RPMS that's often when the exhaust is often the loudest. 

[r0tor]

Our Mazda 3 shows mileage of 99.9 MPG when I take my foot off of the gas and coast in gear.  Tonight I'll try a couple of slow downs coasting in neutral and see what the readout reports.

MY RX-8 reports the same and when looking at the injector pulse width it goes to zilch

[sportyaccordy]

Or they could just not pump fuel to the cylinder, but that would still lean out the a/f mix in the exhaust.

Y? The intake + exhaust valves are closed.