Hand calc vs. DIC mpg question - Chevy and GMC Duramax Diesel Forum
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Old 03-21-2013, 07:00 PM   eBay Motors  #1 (permalink)
CNE
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Hand calc vs. DIC mpg question

So it seems that everyone accepts the DIC fuel mileage calculator to be inaccurate and trusts the hand calculation method. My question is why?

There are known deficiencies with the flow rate measurements on your local gas pump. Temperature plays a role in fluid density which determines how much of that 'gallon' you actually get (you should try to fill up during the coolest part of the day). Also, the faster you pump, the more vapor is created within the fuel. The vapor gives false readings on the flow meter (you should fuel slowly to maximize your 'gallon'). Excessive fuel vapor and foam can also cause the pump to shut off early.

So basically, the system isn't perfect.

Now this is where my real question comes in: What is the precision of the fuel metering device on our trucks. These engines have precision fuel delivery systems, which would make me think that they can monitor fuel usage to extreme accuracy. This would theoretically be able to give a more accurate average and instantaneous fuel mileage report. Is this not the case? Or does GM purposely inflate the numbers (in the ecm) to make people think they are getting better mileage than they are?

Sorry if this has been answered before, but I couldn't find it anywhere.
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Old 03-21-2013, 07:24 PM   eBay Motors  #2 (permalink)
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I have compared the last 18 fillups on my truck and the DIC averages 2 MPG better than actual hand calculate. I have considered changing the tire size in the ppe to compensate but am pretty sure if I do that the speedo will be off. I do not know of a way to calibrate the fuel mileage calculator.
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Old 03-21-2013, 07:28 PM   eBay Motors  #3 (permalink)
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I have similar results, just wondering if anyone has experience with the fuel flow meter on the duramax, that could explain why we generally trust the hand calc over the potentially more accurate DIC.
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Old 03-21-2013, 11:17 PM   eBay Motors  #4 (permalink)
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Sometimes my DIC mpg avg is the same as my hand calc. & when my DIC avg mpg is off, its by 1-1.5 mpg.
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Old 03-21-2013, 11:29 PM   eBay Motors  #5 (permalink)
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snopes.com: Tips on Pumping Gas

I hand calc and monitor DIC every fill up, sometimes they are on some times they aint but they are usually pretty close. Either way i still do both.
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Old 03-21-2013, 11:33 PM   eBay Motors  #6 (permalink)
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My DIC is off, but it is consistantly off by the same amount. So I do go by it, I just add 2.5 mpg and it is accurate.
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