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Old 05-22-2008, 08:23 AM   #1
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Question P&G questions (Non-EOC)

As I've mentioned in other threads, I began to P&G last week. I'm not going to EOC, just idle in neutral, for reasons that I explained elsewhere, so these questions are in that context.

1. Big chunks, medium bites, or small nibbles? What's better? Should I do huge pulses and long glides when possible, or should I always do smaller cycles?

2. If I'm on a long glide and I'm going to come to a stop soon but my speed is getting very low and it's going to take a long time idling until I get there...should I give it another small pulse? I had a situation where I think a 2 second pulse could have saved 10 seconds idling.

3. Is P&G going to put premature wear on my 5th gear synchro? I'm not doing P&G on the highway due to the excessively low gearing of my car, which requires me to do 3000 rpm rev-matching (and which I believe caused the lackluster performance of my first P&G tank, which barely beat my previous non-P&G record by a fraction of a MPG). However, that same low gearing means that sometimes I have up to a 2000 rpm difference for the snychro to make up (or else I have to rev-match before the shift and hold it while I shift). This car is a lease and warrantied so I'm not too worried about it, and I'll be returning it before the warranty is up...

4. Should I choose a route with fewer stops, or shorter distance? There is a portion of my commute where I can go .3 miles with one extra stop, or .57 miles without that stop. The longer route has a longer glide that partially makes up for its distance, I think.
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Old 05-22-2008, 09:12 AM   #2
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holy cow,

first of all, thanks for the advice and encouragement to do this. I have been doing it about a week as well (started monday). I am at about 150 miles on this tank and my average is at 38.8MPG. my previous best tank has been 36.5 and it may have been a little high due to pump error.

I have found that the best gains for me are #1 on really long and steep hills, and #2 coming up to a stop light. if I know the light is going to catch me or if it already has. I can make up some of what it is going to take to get me moving again.

I look at the length of the glide and speed more as a courtesy issue. if there is no one around and you may drop a few MPH then who cares. I try to stay no more than 5MPH under if someone is behind me (they usually still blow by me pretty soon).

as for the small pulse you were talking about giving it. I would say to keep an eye on the scangauge and if you are above your tank average (or where you feel you should be) then keep on gliding.

as far as how large of a chunk you should take, I would say the courtesy thing may apply. also comfort level. I personally am not too comfortable doing 15,20,30 mph under the speed limit.

as far as wear on the tranny, that is a huge concern of mine as well.


I know my opinion of this subject has changed drastically since I have been on this site and hopefully it will pay off. I am going to be very curious to see what this tank's average is for me. I am still very much like you, trying to see where the limits are without permanently damaging my car.
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Old 05-22-2008, 09:33 AM   #3
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Courtesy first - good idea.

Then, as conditions permit - I think the bigger the spread the better. I like 15-20 mph if possible. There is a slight fuel cost each time you shut down or restart the engine, so reducing those events will help. With the engine running, though, there's less of that issue. Even there, there is the time taken for the engine to settle into idle. As small as 5mph spread should help, but not as much. I'd say try for 10mph spread if your engine is running, 15 if you EOC.
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Old 05-22-2008, 09:57 AM   #4
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Quote:
Originally Posted by BEEF View Post
first of all, thanks for the advice and encouragement to do this. I have been doing it about a week as well (started monday). I am at about 150 miles on this tank and my average is at 38.8MPG. my previous best tank has been 36.5 and it may have been a little high due to pump error.
I'm happy to help, and very happy that it really worked for you. I'm sure my current tank will do much better, but now results take longer because I'm driving my truck two days per week instead of the car.

Quote:
as for the small pulse you were talking about giving it. I would say to keep an eye on the scangauge and if you are above your tank average (or where you feel you should be) then keep on gliding.
That would be a great way to look at it. Now if I only HAD a ScanGauge.

Does the ScanGauge allow you to have two data/settings sets, so when I take it from my car to my truck I can easily switch to different vehicle-specific settings and have different FE averages for both?

I spent a few minutes looking into getting the OEM MPG displays that came in more expensive versions of my vehicles, but each one would cost more than a ScanGauge, and the SG is also more useful...

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Originally Posted by PaleMelanesian View Post
Then, as conditions permit - I think the bigger the spread the better. I like 15-20 mph if possible.
That describes the upper end of what I'd describe as "medium". I knew I was using vague terms and should have given some numbers!

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Even there, there is the time taken for the engine to settle into idle.
That's a good point. In my VW, except for its rev hang "feature", it drops to idle very fast; but my GMC has to settle slowly into it.

I do keep everything within the limits of courtesy; even at that, I worry about what's going on in other drivers' heads when they see me gaining and losing ground repeatedly.
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Old 05-22-2008, 10:02 AM   #5
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Originally Posted by theholycow View Post
Should I do huge pulses and long glides when possible
Just to add to what pale said. I think long cycles are better. Another reason is that it's less wear on various parts, like shift linkages, and your right arm and left leg.

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should I give it another small pulse?
I think it would depend on the exact details of the situation, which I don't quite grasp. But my hunch is that the small pulse is not worth bothering with, in the situation you described.

Quote:
Is P&G going to put premature wear on my 5th gear synchro?
I just do some moderate rev-matching. But on my car (VX), 65 mph is less than 2200 rpm, so it doesn't take much.

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Should I choose a route with fewer stops, or shorter distance?
I think moving a car from a dead stop takes a lot of fuel. So the distance might be worth it. But it would take more complex data and calculations to really know.
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Old 05-22-2008, 10:18 AM   #6
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Quote:
Originally Posted by theholycow View Post
4. Should I choose a route with fewer stops, or shorter distance? There is a portion of my commute where I can go .3 miles with one extra stop, or .57 miles without that stop. The longer route has a longer glide that partially makes up for its distance, I think.
I'd take the one that uses less fuel. That'll be hard to know without some kind of realtime gauge, though. From your description, I'd choose the longer one.
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Old 05-22-2008, 10:32 AM   #7
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holy cow,

I think of the scangauge as a "for indication only" device. you have to input the displacement of your engine and fuel tank size but that's it. I wouldn't even worry about the tank size if you were going to swap it from vehicle to vehicle.

there is also a percentage of error that you put in for accuracy. there is a procedure in the book for it. I would think that if you were to figure the percentage of error for the two vehicles then it would be easy to swap from vehicle to vehicle. you could put in the liters of displacement and the percentage of error and you should be good to go.

my scangauge is set up with instantaneous MPG and trip MPG which resets after the ignition is off for 3 minutes (according to the manual). I can see my tank average but I have to go to the trip function to see that.

If you decide to get one (and they are worth every penny) I would suggest that you get the extra cord. I routed the cord in my car so it is quite a bear to get it out to put in something else. got mine from www.thinkgeek.com they used to promote that site here for it. now some water gass stuff has taken its place.

I wouldn't take the scangauges accuracy over real world calculations (thus the error percent) the numbers are only as good as your calibration. it's good to see the numbers for indication but they have been wrong before for me. the calibration made all the difference. I also say to check up on the calibration from time to time. when I first plugged it in I was only getting 24MPG and my calculated was 30 flat.
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Old 05-22-2008, 10:44 AM   #8
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I wouldn't take the scangauges accuracy over real world calculations (thus the error percent) the numbers are only as good as your calibration. it's good to see the numbers for indication but they have been wrong before for me.
A basic limitation of the SG is that it calculates fuel use indirectly (via airflow sensors) rather than reading it directly via a flow sensor or via the injectors. In certain situations on certain cars, this can cause problems with accuracy. But I think many people get satisfactory results, despite this.
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Old 05-22-2008, 12:44 PM   #9
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don't get me wrong, I really like my scangauge. it just has to be properly set up before it can be accurate (relatively accurate). mine is reading pretty accurate now. I cycled through the calibration procedure a few times before I got it there but now it is pretty good.

and the fact that a calibration procedure is a tank of gas (a week for me) made it seem like forever and I didn't want to just drive so that I could calibrate it faster. that would have defeated the purpose.

it was one of the first things I bought for my car for FE. I like it a lot and would recomend it to anyone. my point was that accuracy lies more in mileage and gallons used and not so much relying on what the scangauge (or any gauge for that matter) tells you.
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