Hypermiling Question

[JohnnyRun]

One of the Hypermiling tips, number 76 to be exact, says to switch to neutral when stopped (in an automatic vehicle) to avoid the car trying to creep forward. This makes sense to me, but I've noticed (in 2 cars, an Acura TL and a VW Beetle), that when I am stopped and shift to neutral, the RPM's climb ever so slightly. That got me wondering if that climb in RPM's offsets the amount of fuel saved in shifting to neutral in the first place. Any ideas?
Thanks guys, I'm just curious :P

[UFO]

My Scangauge tells me fuel consumed idling in neutral vs in drive is roughly half. I bet that is common to most automatics. It's the engine load that consumes fuel, and a small rpm spike does not take much.

[Frank Lee]

Plus if there's brake lights on vs off...

[metroschultz]

Originally Posted by UFO

My Scangauge tells me fuel consumed idling in neutral vs in drive is roughly half. I bet that is common to most automatics. It's the engine load that consumes fuel, and a small rpm spike does not take much.

My wife and I had this same discussion in her Avalon.
I was able to show her on the SGII that at idle, in neutral fuel consumption is .4 gph. In drive, at idle fuel consumption was .75 gph.
Almost twice as much fuel burned per hour at idle in drive v neutral.
Even though the idle went up by 100 rpm.

[saand]

It seems to depend how your car controls its idle. In my relatively old auto mazda 626 91 year without any modification it would use up the same or a little bit more fuel if i put it into neutral. This was because the idle went up with the reduced engine load.
If the car actively controls the idle RPMs then the car will use less fuel when idling in neutral compared to drive.
The best way you can tell is to use a scan gauge or similar device.

As a general rule of thumb (that i am making up right now) if your idle goes up by more than about 50 rpm its likely your car will use the same or more fuel when idling in neutral compared to drive.

Note: some cars can be put into a "idle tuning" mode where the idle control will be turned off or set to a particular level. If your able to do this you can manually tune the idle with the set screw so that when you turn off the engine idle control, the car will idle as low as you set it. This works only when you can place a switch for this in the engine cabin otherwise the engine might stall with low RPM when in drive at the lights

[Frank Lee]

Why do the mfg's set the idle the way they do, when they have complete control of it?

[capnbass91]

My check engine light came on and its cuz the idle throttle is too high, made a big difference in my mpg. when i put it in neutral at a stop it went up about 300-400 rpm. Getting that fixed thursday! so much wasted gas >X(

[JohnnyRun]

Woah, thanks for all the responses guys! I'm definitely looking into getting a ScanGauge or something similar. Which would you guys recommend? I'm taking price and function into consideration because ScanGauge's seem pretty expensive to me, and I saw an ad on this site for one (I forget the name), but was like 50 dollars. Any tips from people who've used both?

[vskid3]

Ultragauge is probably what you're thinking of. I have one and I like it. If cheap is best for you and there isn't something specific you need from the scangauge (like a certain x-gauge), I would highly recommend it.

[JohnnyRun]

That sounds really good, I just wanna be able to raise my mpg's haha. Does the Ultragauge allow me to see trip MPG and best MPG? Thats what I think is most important, right?