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Old 08-13-2007, 05:18 PM   #11
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Originally Posted by veloman View Post
What if one were to somehow monitor the speed of the car's exhaust? Would this correlate well enough with fuel consumption? It wouldn't be hard to measure.
Hmm... I'll have to think about that, interesting...
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Old 08-13-2007, 05:26 PM   #12
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You would think someone would come out with a MPG meter, for much less money. You could sell many.
Once you have the basic interface and processor in place, any additional functionality is "just ones and zeros." Excluding possibly needing more RAM or processor power, it's a one-time development cost.

Thus I'd expect any sort of ODB-based MPG display would come in around the same price point as a ScanGauge. Having developed similar hardware (in the sense of being small and having an embedded processor), I'm pretty confident the ScanGauge is price fairly. For an MPG-only display you could save a little with a smaller display (or maybe even an analog readout), you could use a smaller/less powerful processor, you could get them built overseas where labor is cheaper, but overall I doubt there'd be tons of opportunity to cut costs. And I suspect the percentage of drivers willing to cable up to their OBD port and mount a readout to watch MPG is pretty small, so economies of scale probably wouldn't be available.

Seems like you're left with reading fuel injector and speedo pulses (which requires more technical savvy and effort to hook up, thus shrinking your market even more), falling back to the old vacuum gauge trick, or something along those lines.

So while I'd love to see an accurate MPG meter available for $39.95 at Wally World, I can understand why it hasn't happened (and might not).

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Old 08-13-2007, 06:52 PM   #13
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"So while I'd love to see an accurate MPG meter available for $39.95 at Wally World, I can understand why it hasn't happened (and might not)."
I appreciate your thoughts on why there is not something simple and cheaper out there.
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Old 08-13-2007, 07:09 PM   #14
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Hmm... I'll have to think about that, interesting...
Well, after some tinkering with materials, I've decided to scrap that idea. Instead, I will measure how much my gas pedal is depressed and then factor in current speed to get a "mpg" or FE reading. I don't need to actually know what my exact current mpg's are, all I really want to know is what throttle position is most efficent.
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Old 08-13-2007, 07:12 PM   #15
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...all I really want to know is what throttle position is most efficent.
Most likely the least amount necessary to keep your speed. I find it to be 12-14% in my car on flat ground.
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Old 08-13-2007, 07:21 PM   #16
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I am probably going to open my mouth and put my foot in it but here goes.

Somewhere on the net I read that you could use one of the older ford trip computers from a junk car. You just need to suppy voltage VSS and Injector signal to it. I am not sure you could calibrate it to your car. I could not find the article in a couple minutes looking. Maybe it relates to the earlier link in this thread. Unless you really get off on this kind of stuff it will take less time to get a second job and buy a scangauge II.

There are used trip computers on ebay.

This might be the spot but I did not join. Anyway you get the idea.
http://www.fordmods.com/forums/documents.php?doc=82
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Old 08-13-2007, 07:25 PM   #17
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Originally Posted by roadrunner View Post
I appreciate your thoughts on why there is not something simple and cheaper out there.
Now let's hope someone posts on where to find that $39.95 MPG gauge... this is a case where I'd love to be proven wrong!

Rick
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Old 08-13-2007, 07:37 PM   #18
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I just bought an oil pressure gauge for my Integra for $37.99 and it gave me an idea to develop a way to convert it to an mpg gauge. The readout is perfect, 0-105psi will become 0-105mpg.

I have all the circuitry to convert the tach, Vss, and injector pulses to voltage outputs and using op amps will be able to divide mph by amount of fuel used. The circuitry will be inexpensive, probably less then $25. The most expensive thing unfortunately will be the 2 DC/DC converters (+5v, and +/- 12 volts the op amps need) These are about $10-15 each which will add to the cost. It will only be an instant reading, no averaging.

I had hoped the sending unit for the oil pressure gauge sent a voltage input to the guage, but it is a proportional resitance (3-163 ohms). For this, I'm going to have to talk to one of the EEs at work for a little help in getting a digital potentiometer to work, but I think it's doable. When I get a little further, I'll make a thread on it.
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Old 08-13-2007, 07:48 PM   #19
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...Somewhere on the net I read that you could use one of the older ford trip computers from a junk car. You just need to suppy voltage VSS and Injector signal to it...
Here is a real life gassavers user doing basically the same thing, found an old mpg meter in the yard and installed it in his car:
http://www.gassavers.org/showthread.php?t=4258
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Old 08-14-2007, 03:36 AM   #20
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A combination of a vacuum gauge and mv reading of the O2 sensor would give you two data streams...throttle position and closed loop?

By utilizing your free and unused human brain to process this information...you have a DIY mpg meter.
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