Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - Scanguage says 25mpg, but manual measurement says = 35mpg

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Scanguage says 25mpg, but manual measurement says = 35mpg

If I measure from top of tank to top of tank, plug in the cost and the distance I travel I get 35mpg for my van.

However, if I plug in my scangauge i get 25mpg max... I never see it rise to 35...
I have the wheel diameter set correctly and the same engine size (2,5 turbo...)

You need to calibrate your Scan Guage. Next time you fill up you can enter the amount of gallons/liters used into the SGII to calibrate it.

Quote:

Originally Posted by Angmaar (Post 206348)

You need to calibrate your Scan Guage. Next time you fill up you can enter the amount of gallons/liters used into the SGII to calibrate it.

yep this is what you need to do. Do it for two tanks and it should be very close.

It will never be right on but will stay within a % or 2.

I disagree: the Scangauge just isn't very good at measuring fuel consumption. I have one in my Insight, which also has the factory mpg gauge. The factory one is as close to accurate as I can measure, the Scangauge reading often differs by 20 mpg or more. That's when it's calibrated, and it loses calibration very easily.

Quote:

Originally Posted by jamesqf (Post 206548)

The Insight has weird A/F ratios (up to 1:25 or something) and the SGII was designed for 1:14.7. So it won't be accurate in less you don't drive in lean burn mode.

Mine is consistently accurate within 1% once calibrated.

...my SGII, on my SIDI '09 Vibe, seems to run within ±2% fairly consistantly, and that's with the Toyota Spark-Ignition Direct Injection (SIDI), not normal MFI.

jago25_98 -

I would venture to guess that the SG is not as good at turbo engines.

Here's a silly question but I have to ask it. Is the fuel set to US gallons or Imperial gallons?

CarloSW2

Quote:

Originally Posted by Angmaar (Post 206560)

The Insight has weird A/F ratios (up to 1:25 or something) and the SGII was designed for 1:14.7.

Yeah, that's the problem (at least as I understand it): the SGII isn't actually measuring fuel consumption directly, it's inferring it from other measurements and some assumptions, like the A/F ratio. If your engine doesn't follow the assumptions it uses, the values will be wrong.

There are other cars besides the Insight that have some variety of lean burn tech: Lean burn - Wikipedia, the free encyclopedia

This is what happens to a algorithm that uses pulse width, along with other engine measurements like rpm, tps, map/maf, O2. It is making inferences, and not measuring actual flow of fuel- ccm/m. If it knew the size of the injector (fuel flow capability) and the pulse width, it would be more likely to be accurate with lean burn. Just my thoughts.

jamesqf -

Quote:

Originally Posted by jamesqf (Post 206623)

Yeah, that's the problem (at least as I understand it): the SGII isn't actually measuring fuel consumption directly, it's inferring it from other measurements and some assumptions, like the A/F ratio. If your engine doesn't follow the assumptions it uses, the values will be wrong.

There are other cars besides the Insight that have some variety of lean burn tech: Lean burn - Wikipedia, the free encyclopedia

I agree. It is my claim that the SG algorithm is very similar to this algorithm which assumes 14.7 AFR :

Calculating MPG from VSS and MAF from OBD2 - MP3Car.com - 09-21-2007

Quote:

... For the record, the first "one-line" MPG formula above, taken from my Circuit Cellar article, is off by 100! The "4.54" should in fact be "454". The correct formula is:

MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100)
MPG = 710.7 * VSS / MAF

Note that OBD-II VSS reading is in kilometers/hour and MAF reading is grams/sec times 100.

This formula works very well in a modern automobile because the engine computer spends almost 100% of its time managing the fuel-air-ratio to 14.7, which it can do very well because of the "closed loop" feedback from the O2 sensor(s).

In fact, the accuracy of this method has been proven in literally tens of thousands of gasoline-powered vehicles. Accuracy within a few percent is typical, often limited by the accuracy of the vehicle speed reading (i.e., VSS).

As for other ways of doing this, especially if you don't have a MAF sensor, by knowing the displacement of the engine, and after a simple "calibration" using fuel tank "fill-up" data to find the only unknown, namely the "volumetric efficiency" (VE) of the engine, MAF can be calculated from RPM, MAP and IAT. With VE, one can use the following formulas to calculate a synthetic "mass air-flow" (MAF) in grams per second, all without a MAF sensor, using the "Ideal Gas Law", as follows:

IMAP = RPM * MAP / IAT
MAF = (IMAP/120)*(VE/100)*(ED)*(MM)/(R)

where manifold absolute pressure (MAP) is in kPa, intake air temp (IAT) is in degrees Kelvin, R is 8.314 J/°K/mole and the average molecular mass of air (MM) is 28.97 g/mole. Note that, in the above formula, the volumetric efficiency of the (4-cycle!) engine is measured in percent and the engine displacement (ED) is in liters.

The VE of my 1999 7.4L Chevy Suburban is about 65%. Smaller, higher performance engines can have VE's of 85% or higher.

...

CarloSW2

My Scangauge has been calibrated and accurate for several years now. My cumulative error is about 1/2 gallon, TOTAL.

True, it doesn't calculate lean burn, but most cars don't have that feature. For those that run standard AFR, it's quick and easy and accurate.