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Old 02-16-2007, 07:26 PM   #11
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Originally Posted by Mike T View Post
...
So how's the braking with engine off, on the second or third application? ..
Mine is stiff but useable. One *could* increase the vacuum capacity of the system simply and lightly enough, and even add a pressure (vacuum) switch to sound a warning when it's time to recharge the brake vacuum.
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Old 02-16-2007, 08:36 PM   #12
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ok, I appreciate that the precision isn't terribly justified here but:
return trip 1 = .2161 gallons for 5.1mi
return trip 2 = .2581 gallons for 8.7mi

Therefore the mpg to get back to the spot where test one ended =
(8.7mi - 5.1mi)/(.2581 - .2161) = 85.7mpg for 3.6 miles (uphill)?

I think there's more to those uncovered 3.6 miles in test 1 than implied.
Cheez!! I thought this to be a very simple test with obvious advantages. A free gift. You invented the question, what is your answer?

It seems that if I tried to pass out free $100 to each passerby, some could not decide if they wanted cash, check or money order.
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Old 02-16-2007, 09:04 PM   #13
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Cheez!! I thought this to be a very simple test with obvious advantages...
Like I said, I appreciate anyone who is willing to test and post their results.

You coasted 3.6 miles farther in test 2, that's the only specific that one can conclude from the test though. Saying +20mpg is not a conclusion that can be reasonably drawn from this test since the vehicles did not cover the same course, the first one was substantially shorter.

My corrections are gratis also
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Old 02-16-2007, 09:46 PM   #14
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Brakes!

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Originally Posted by Mike T View Post
I agree: bike!

So how's the braking with engine off, on the second or third application? does the car have ABS?
Lately when I have been coasting with the engine off, I have been using the emergency hand brake to "adjust" my speed, saving the "real" vacuum assisted brakes for an "emergency" stop, if I need them.
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Old 02-16-2007, 10:07 PM   #15
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An extension of these tests would be to test the uphills. Do you get better FE if you accelerate on the flat at the bottom of the hill to build your speed up before reaching the bottom of the hill so you can get further up the hill before having to drop into a lower gear, or is it better to maintain the lower aero drag of your normal cruising speed and accept having to drop into a higher fuel consuming lower gear earlier on the uphill?
I like to look at what the fuel is used for. When you drive fast a larger portion of fuel usage is used to overcome aero drag, which you'll never get back. When you climb a hill the fuel is being used to create potential energy (mass of car raised up to a height) which you have a chance to recoup when you coast down the hill. Hopefully you won't have to brake on the downhill and waste the fuel. I don't think it's bad to have to downshift on a climb. To me this means you're still converting fuel into potential energy as quickly as the car is capable of. Just make sure you don't go into open loop or rev the engine higher than needed.
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Old 02-16-2007, 11:32 PM   #16
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The single most important improvement to FE you can ever do. FREE

Walk or bike
As long as we're just being silly, wonder if anyone has thought of a car rack for a bike?
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Old 02-17-2007, 12:30 AM   #17
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skewbe -

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Hey CO, good on ya for testing, but something doesn't add up. No argument that you got 3.6 free miles on the second test in one direction. But the MPG differences might be misleading.

Since you used no fuel on the way down, the climb back up MPG is 1/2 the total MPG. Trip 1 = 23.6 mpg on the climb, Trip 2 = 33.7mpg on the climb.

Given this (and the drag of the engine) it seems apparent that the car stopped where the average return trip is significantly steeper in test 1.

I think you would have to cover the same course/distance (i.e. enable the ignition a couple times on test 1) in order to get an accurate comparison.

Had you slammed on the brakes and turned around at 5.1 miles in test 2, the mpg results would have been identical, the question is how much extra fuel does it take to extend test1 to cover the same ground as test2?
I agree that covering the same distance would be a better comparative test, but I don't think the reported MPG would necessarily be the same, at least not for my Saturn.

On the ScanGauge, with my engine off, coasting in Neutral, I still see 0.1 GPH (Gallons Per Hour) consumption. This must be a fudge number to avoid divide-by-zero errors for when the fuel injectors are off. This fudge number leads to *another* silly value I see in the ScanGauge. When the engine is off, at X MPH, with the key in the run position, I see X*10 instantaneous MPG, i.e. 39 MPH yields 390 MPG. In this context it should be the "infinity limit" of the SG, or something like 9999 MPG. But 390 MPG now makes sense to me because :

39 MPH / 0.1 GPH = 390 MPG!!!!!

If CO ZX2 covers the same ground (5.1) miles in less time and his ZX2 talks to the ScanGauge in a similar manner (I can test this, my Dad has a ZX2), the 0.1 GPH will be divided into a smaller portion of time when the downhill MPH is higher and result in greater total MPG for the Engine-Off-Neutral test. Let's plug in the example numbers, assuming that the 56 MPH average would also apply for the first 5.1 miles (this example would apply for my Saturn in relation to the ScanGauge) :

Engine Off in Gear
36 Miles/Hour for 5.1 Miles => 5.1 / 36 = 0.142 Hours
0.142 Hours * 0.1 GPH => 0.014 Pseudo-Gallons

Engine Off in Neutral
56 Miles/Hour for 5.1 Miles => 5.1 / 56 = 0.091 Hours
0.091 Hours * 0.1 GPH => 0.009 Pseudo-Gallons

The second test over 5.1 miles will yield a higher MPG for the round trip because the Scangauge is fudging the engine off numbers. The ScanGauge, when confronted with the infinite (MPG), goes a little loopy, .

This is a defect in the ScanGauge software, *or* a legitimate compromise in a context where the ScanGauge is not designed to operate, depending on your POV.

Tentative Conclusion : If CO ZX2's car is reporting 0.1 GPH in Engine-Off Coasting mode, he is actually getting even better downhill MPG than he is reporting.

When the engine is off, all bets are off regarding ScanGauge accuracy. Different ECU/PCMs are saying different things to the ScanGauge.

The ScanGauge is good for comparing apples to apples, Saturns to Saturns, Geos to Geos, etc. ad-nauseum ...... It's also good for comparing relative gains of your own car, but the gaslog tells the real truth.

YMW(ill)V!

Orrrrrrrrr, how 'bout them apples!

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Old 02-17-2007, 01:02 AM   #18
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hand brake to slow down

hand brake to slow down that is a great idea I did not think of that becaust it seems so unnatural. I do alot of engine off coasting I'm going to start trying that to save my power brakes for when I really need to stop fast.
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Old 02-17-2007, 06:30 AM   #19
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Tentative Conclusion : If CO ZX2's car is reporting 0.1 GPH in Engine-Off Coasting mode, he is actually getting even better downhill MPG than he is reporting.

When the engine is off, all bets are off regarding ScanGauge accuracy. Different ECU/PCMs are saying different things to the ScanGauge.

The ScanGauge is good for comparing apples to apples, Saturns to Saturns, Geos to Geos, etc. ad-nauseum ...... It's also good for comparing relative gains of your own car, but the gaslog tells the real truth.

YMW(ill)V!

Orrrrrrrrr, how 'bout them apples!

CarloSW2
This is the conclusion I came to soon after I got my first SG. The SG is a great tool but, as great as it is, there are so many variables involved that you can't get an absolutely 100% accurate reading ALL the time. You can get it dialed in pretty close but you'll never always be dead on. I use the SG for measuring improvement over different segments and the hand calcs when topping off the tank as the final word. As for those who were criticising CO XX2 recently, Carlos is right, when you analyze things as carefully as Carlos has, CO ZX2 is doing even better than he was reporting. It's all about the commute conditions.
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Old 02-17-2007, 06:30 AM   #20
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skewbe -

I agree that covering the same distance would be a better comparative test, but I don't think the reported MPG would necessarily be the same, at least not for my Saturn...
Yah, on some cars (i.e. mine, apples if you like) the scangauge reports base fuel consumption during overrun/EOC in gear, even there isn't any, which is not a big deal IMHO.

CO reported zero fuel used at the end of his first test leg so I assume his doesn't do that.

Just a thought, A carbureted car very likely would still spew fuel into the engine when coasting with in-gear coasting with the ignition off , and make a nice backfire when you turned it back on
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