Got my Scangauge-pretty eye opening - Fuelly Forums

Android Users - Coming Soon! - Migrating from aCar 4.8 to 5.0

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Old 05-22-2008, 07:43 AM   #1
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Got my Scangauge-pretty eye opening

As I was just posting somewhere else, it's amazing how insightful this little doo-dad is. Oh man, all I can say is I'll never use cruise on anything but level ground again. Watching it on the scangauge, the difference was when it was on, going uphill I'd drop to around 9 mpg even though I was still maintaining vac and not slipping to turbo boost. Off, just letting the car lose a few mph, I was able to keep the instant miles in the high 20's/low 30's. Unreal.
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Old 05-22-2008, 09:50 AM   #2
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That's very interesting. Is your car automatic or a stick? If the former, did it switch to a higher gear when you eased off the throttle? Just curious.
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Old 05-22-2008, 09:59 AM   #3
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Originally Posted by bkrell View Post
Watching it on the scangauge, the difference was when it was on, going uphill I'd drop to around 9 mpg even though I was still maintaining vac and not slipping to turbo boost. Off, just letting the car lose a few mph, I was able to keep the instant miles in the high 20's/low 30's. Unreal.
I've noticed that effect too.. Efficiency plummets when you try to keep a constant speed going uphill. I've found that gradually slowing going uphill, a slight speedup when starting downhill then coast until near the bottom and an acceleration near the very bottom or on the flat between hills seem to boost overall efficiency the most.

Some kind of "adaptive cruise control" that would read engine load and efficiency as well as speed would be very useful.
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Old 05-22-2008, 10:13 AM   #4
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Beware the instantaneous MPG leading you astray, it's not as efficient to accelerate at 23mpg for 2 miles when you could accelerate at 15mpg for 1/4 mile to get up to your 28mpg cruise....

.01666g +.0625g < .0869g
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Old 05-22-2008, 10:23 AM   #5
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That's very interesting. Is your car automatic or a stick? If the former, did it switch to a higher gear when you eased off the throttle? Just curious.
auto. Stayed in 4th (O/D) the whole time.
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Old 05-22-2008, 10:29 AM   #6
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Beware the instantaneous MPG leading you astray, it's not as efficient to accelerate at 23mpg for 2 miles when you could accelerate at 15mpg for 1/4 mile to get up to your 28mpg cruise....

.01666g +.0625g < .0869g
Right, but I was just talking about hills and momentum. Good point, though
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Old 05-22-2008, 10:40 AM   #7
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auto. Stayed in 4th (O/D) the whole time.
Thanks for answering the question.

Road is making a good point. Reading instantaneous mpg can be fundamentally misleading. That's because the SG (or similar instrument) is taking into account how much distance the car is covering, but it's not taking into account work that's being added to the vehicle, in the form of potential and kinetic energy. That energy will pay you back later. And it's also not taking into account the duration of the energy use (which is basically what Road was saying, I think).

That's why it's important to make comparisons based on an entire trip, and not just a series of instantaneous readings.

And these issues are separate from other accuracy issues that come up with something like the SG, associated with the fact that it's not directly monitoring the injectors.

It's still a great tool, but it helps to understand the limitations.
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Old 05-22-2008, 10:51 AM   #8
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Right, but I'm not talking about prolonged situations. I'm talking about dropping a few mph on the upslope instead of maintaining constant speed. Doesn't that make sense or am I wrong?
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Old 05-22-2008, 11:13 AM   #9
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Doesn't that make sense or am I wrong?
Good question. I don't know. What I'm saying is that the math is deceptively complicated.

Here's an example of what I'm talking about. Consider this comparison:

A) Travel up the hill at a steady 25 mph
B) Travel up the hill at a steady 30 mph

Contrast that this with comparison:

C) Travel up the hill at about 24-26 mph, but with the speed slowly dropping
D) Travel up the hill at a steady 30 mph

When an instrument like the SG looks at C, it's going to give you a deceptively optimistic result. Why? Because it's calculating mpg based on fuel use and distance, but it's not taking into account the fact that you're losing speed. The momentum you're losing was paid for previously, by gas you burned a little earlier. And now you're relying on that momentum, instead of fuel, to propel the car. That's why the car is slowing down.

The reverse happens when you're accelerating. The SG will give you a deceptively pessimistic result. It's going to tell you that you're burning a lot of fuel relative to the distance you're traveling, but it's failing to take into account that some of the fuel you're burning is being used to create momentum in the vehicle. It's like putting money in the bank. In a few moments, you'll be relying on that momentum.

When you add speed, you're adding kinetic energy, and when you climb a hill, you're adding potential energy. Either way, you get this energy back later. But the SG is not taking this into account, if you're just looking at an instantaneous reading. That's why you have look at a broader interval of analysis, like a trip.
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Old 05-22-2008, 11:27 AM   #10
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Gotcha. I already have a resettable trip gauge on the car's computer. This is what I've always relied on. I understand what you're saying, but my question is this...as long as I am staying in OD gear, wouldn't it still make better sense to use momentum even if you get deceleration that you have to make up somewhat on the downslope? I mean, aren't you loading the engine more by keeping steady speed on the upslope?

I barely passed physics. That's why I am an archaeologist.
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