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Old 06-17-2008, 10:39 AM   #51
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Quote:
Originally Posted by theholycow View Post
The ScanGauge has to guess/calculate fuel usage. It can't measure GPH, there's nothing in the OBDII standard for that.
I'm pretty sure this is exactly true, but I want to add something to help people not get confused.

There are scantools that mechanics use, that plug into your OBDII port and report a lot of things. Some of the scantools report injector data. This seems to contradict what you said, but not really. I believe the folks who made the scantool paid a licensing fee to one or more car makers, and were given data about the proprietary method that maker uses to report injector data via the OBD port.

In other words, the data is not part of the OBDII standard, and therefore it's not easy to get at, even though it does exist, on some cars.

I think Scangauge decided to not to try to get at the data that way, presumably because it would have been difficult and added expense to the product. Maybe a lot of expense. I wouldn't be surprised if the car makers demand a licensing fee on a per-unit basis.
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Old 06-17-2008, 10:43 AM   #52
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Originally Posted by PaleMelanesian View Post
I dunno. That's just the first search result I found, and it doesn't look very reputable.
A lot of cd data is here:

http://web.archive.org/web/200412040...rmayf/tbls.htm

I don't see the CRV, but maybe there's some similar vehicle, to make a comparison.
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Old 06-17-2008, 11:38 AM   #53
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Quote:
Originally Posted by monroe74 View Post
I think Scangauge decided to not to try to get at the data that way, presumably because it would have been difficult and added expense to the product. Maybe a lot of expense. I wouldn't be surprised if the car makers demand a licensing fee on a per-unit basis.
You are most likely correct. It seems extremely expensive to get manufacturers' proprietary stuff like that.

And, here's proof that the SG definitely calculates/guesses fuel rate:
Quote:
Originally Posted by Project84 View Post
I EOC'd today on my commute and w/ engine off it was still reading 0.07 GPH and at 45mph it was only 477mpg.... this is with engine off!
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Old 06-17-2008, 12:16 PM   #54
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[QUOTE=monroe74;106420]A lot of cd data is here:

http://web.archive.org/web/200412040...rmayf/tbls.htm
QUOTE]

Cool!
The Del Sol has a Cd of 0.42, but tiny frontal area. Could work on that!

The Ford Explorer has a Cd of 0.45, in line with what I found floating around for a Gen'1 CR-V, reported above.
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Old 06-17-2008, 02:26 PM   #55
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Originally Posted by PaleMelanesian View Post
and aerodynamic drag is proportional to the CUBE of velocity.
How do you figure that?

drag = 1/2 rho Cd S V^2

rho = air density

Cd = drag coefficient

S = area

V = velocity
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Old 06-17-2008, 03:04 PM   #56
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In metric system:

rho: kg/m3
S: m2
V: m/s

Therefore drag = kg.m/s2 Are these the right units?




The small v is just a unit vector that points the direction of movement in 3D space, doesn't change our computation for our application, rho=1.293 kg/m3. A means area, same as S (for surface) above.

from http://en.wikipedia.org/wiki/Drag_%28physics%29

Bottom line: 2X speed = 4X drag force, 3X = 9X force etc. Squared indeed. However, the power dissipated seems to be cubed... but you have to take into account the total time to travel a fixed distance, to figure out your net energy cost (you cube but then you actually go faster on that distance). Argh help.

BTW:

If you know the drag % of the underbody (or the wheels), and the drag of an air dam, you could tell if adding an air dam is likely to provide any benefits (increasing the frontal surface S, aka A, but reduces Cd by some amount)...
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Old 06-17-2008, 08:04 PM   #57
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Originally Posted by sonyhome View Post
Bottom line: 2X speed = 4X drag force, 3X = 9X force etc. Squared indeed. However, the power dissipated seems to be cubed... but you have to take into account the total time to travel a fixed distance, to figure out your net energy cost (you cube but then you actually go faster on that distance). Argh help.
Yes, if I'd pulled my head out of my arse I would have taken the next step and calculated the work equation. Pale is correct. I'm a dullard. D'oh.


Quote:
If you know the drag % of the underbody (or the wheels), and the drag of an air dam, you could tell if adding an air dam is likely to provide any benefits (increasing the frontal surface S, aka A, but reduces Cd by some amount)...
I think it's a bit more complicated than that, since the drag is a function of the entire body, and the interactions between the components is not linear. But I think your point has merit....
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Old 07-27-2008, 07:30 PM   #58
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Time to wake up this thread.

So I did a P&G experiment on my comute:

- Commute: 3 miles one way, with lights, 35PMH limit surface streets, return often with 1.5miles highway (next exit) and lights & stop sign. Night twice a week, drive to the gym, about 10 miles R/T surface street.

Experiment result

- Old method: Fast acceleration from light, 1st/2nd/3rd then coast down, 3rd or 4rth gear short pulse, then glide. Result is accelerations are short followed by long glides. Shifting usualy around 2500RPMs.

Result: 35MPG over 338 miles

- New method: Slow acceleration from light, 1st/2nd/3rd/4rth/5th, fast shifting to stay below 1500rpms, but anemic acceleration. I don't really do WOT acceleration, but my foot is pretty heavy after the 1st gear. End result is I spend more time accelerating than coasting.

Result: 35.22MPG over 191 miles

Bottom line:
Seems like both techniques work about the same in terms of fuel economy. However the new method supposedly with better MPGs is harder to drive I find because the car is less responsive. I am more likely to miss a light.

I wonder if I should try one run with normal driving, no P&G. I wonder if I'd fall back sub 30 or not, with somewhat controlled driving conditions. City EPA for my car is 25MPG (new rating) so I'm 40% overjust from P&G and some awareness of lights.
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