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Discussion Starter · #1 ·
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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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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Discussion Starter · #3 ·
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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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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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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My DIC seems to be spot on, every once in awhile, it will be a little higher than what I hand calculate. Has anyone actually seen a DIC show less mpg than hand calc?
 

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Mine is weird sometimes it 2mpg high other times it's 4 mpg high. All I know is its high. Filled up the other day truck said 20 hand calculation was 16.4
 

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Can you apply the same reasoning to the fuel systems in our trucks? Pumps are tested and adjusted once a year (at least here in stl). Fuel is stored at ambient temp in our trucks which temps change depending on time of day. At fuel stations they are in ground at a constant temp year round. I use the same pump for my fill ups and fill up after work in the afternoon. My DIC is consistantly over by .5 mpg. I think both pump and truck are very consistant at least. Pumps actually measure volume. Our trucks dont. Its all based on charts with a hundred variables to tell the computer how long to open an injector. Im not an expert but it makes sense? :cookoo[1]:
 

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Mine is close enough that I haven't done the hand calculation recently. If I put in maybe 20 gallons the DIC will say 19.7 used or something.
Very close.
 

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My DIC seems to be spot on, every once in awhile, it will be a little higher than what I hand calculate. Has anyone actually seen a DIC show less mpg than hand calc?

Mine use to be lower than actual, but that is because my speedo was off. Now it seems to be very close.
 

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Mine DIC always reads 5-8% higher than hand calc. I would believe the pump over the DIC. I know there are state agencies here in CA that randomly check service station pumps for accuracy. The DIC on these trucks use algorithms not physical flow meters to determine fuel used. Algorithms assume a perfectly running truck.
 

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One of the reasons I've come up with for the discrepancy is that the on-board computer only reads a "full" tank as the spec gallons. If you pack the tank after the handle clicks, getting an extra .5 to 1 gallon in the tank, the computer doesn't recognize that, so can't figure it into the calc. What I can't figure out is why they can't design a system that accurately measures fuel to the pump vs fuel through the return and net the actual burn from that and divide by the miles. What the hell do I know? I'm not smart enough to properly use the manual shifter on my '02, so they only gave me the option to shift from OD to 3rd, skipping 4th completely.

Lyle
 

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Avg MPG = miles driven/fuel used. Has nothing to do with how much you refill the tank when at the pump. Doesn't matter if I fill talk half way or all the way full computer could care less.
 

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If you want real accurate MPG, top off the tank and hand calculate.
My dic will vary between .0 to .7, idle time, stop signs/street lights stop and go traffic will affect the accuracy of the dic.
 

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Avg MPG = miles driven/fuel used. Has nothing to do with how much you refill the tank when at the pump. Doesn't matter if I fill talk half way or all the way full computer could care less.
Sure it does. The computer doesn't know the exact gallons you put in; only what the sending unit says. My wife's Genesis Coupe =always= hand-calcs about 1-1.5 mpg less than the computer, but I tend to pack it. The computer "sees" 17.5 gallons when the gauge reads full; but I can put in nearly an extra gallon over that, if I really pack it in. Computer doesn't "know" this. On my D'max, I have 265 tires and, until I got a GPS to verify, I was under-calcing my miles by 4%. That translates into about 10 miles more traveled per fill-up than my odo shows. You might not think it, but it makes a real difference.

Lyle
 

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Be careful now. You're about to cross a thin line between common sense and science!!! :rof
 

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"Fuel used" in the DIC in NOT based on the level of your fuel gauge is my point. It is based on algorithms that continuously calculate a "theoretical" fuel consumption based on various engine parameters. It has no concept of how much fuel is physically in the tank so how much you fill it is irrelevant
 

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If you have bigger tires your MPG will be off. You're traveling farther than the ECM thinks you are. Same can be said with the speedo. At 55 on my speedo, my edge that is adjusted for 315/75 tires says I'm going 62. MPG=fuel used/ mileage traveled. With bigger tires I'm traveling farther than my truck thinks so of course the DIC is off.

My question is if we have wider tires, how much does more fuel is it taking to turn them? Our DIC is accounting for the stock width isn't it?
 
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