Hand calc vs. DIC mpg question - Chevy and GMC Duramax Diesel Forum
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post #1 of 23 (permalink) Old 03-21-2013, 06:00 PM Thread Starter
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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.

2012 Chevy Silverado 2500HD Crew Cab Z71
Nicktane Adapter w/ CAT 1R-0750
Amsoil By-Pass Oil Filtration System
Edge Insight CTS
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post #2 of 23 (permalink) Old 03-21-2013, 06:24 PM
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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.

2006 ccsb lbz 2 wheel drive 4" mbrp no cat Pyro PPE economy
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post #3 of 23 (permalink) Old 03-21-2013, 06:28 PM Thread Starter
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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.

2012 Chevy Silverado 2500HD Crew Cab Z71
Nicktane Adapter w/ CAT 1R-0750
Amsoil By-Pass Oil Filtration System
Edge Insight CTS
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post #4 of 23 (permalink) Old 03-21-2013, 10:17 PM
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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.

2009 GMC Sierra 2500HD SLE CC/SB 4x4 LMM
EFI Live, Edge Insight CTS, Flo~Pro 4" DP back exhaust w/ muffler & stock trumpet tip, Resonator delete plug, Alum. bleeder screw, WIF sensor delete plug, Fumoto oil drain valve, Pacer LED cab lights, GM dually light bar under tailgate, Footwell lights, Cobra 29 WXNWST CB radio, Wilson 2000 CB antenna, Kenwood speakers, GM chrome oval running boards, Bilstein 5100s, Michelin Defender M/S on PY0 wheels, Rubber mud flaps, Curt class IV hitch
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post #5 of 23 (permalink) Old 03-21-2013, 10:29 PM
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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.

2017 GMC Sierra 2500HD SLT CCSB L5P

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post #6 of 23 (permalink) Old 03-21-2013, 10:33 PM
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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.

07' Chevy Silverado LT3 ccsb LBZ. 8" Bulletproof lift kit(junk), Fox shocks, EFI Live by Kory, Edge CTS Insight, 4" Silverline downpipe back exhaust, PPE FPR shims, PPE fuel rail fitting, pcv reroute, resonator plug, Transgo jr, S&B Intake, Amp steps, 20" XD Racing wheels, 36" Goodyear Duratrac tires, Air Lift rear air bags with wireless compressor, billet grill, debadged, Caddy gauges.

77' Ford F-250 factory highboy. 460, hooker headers, 3" flowmaster exhaust, MSD ignition, Comp Cam, Edelbrock high rise intake, Holley 750 cfm double pumper carb, Holley fuel lift pump, Autometer gauges, 3" lift kit, 38" Toyo tires, 18" Weld wheels.
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post #7 of 23 (permalink) Old 06-05-2015, 10:51 AM
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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?


2015 GMC Sierra 2500 Denali HD Crew 4x4 6.6L Duramax
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post #8 of 23 (permalink) Old 06-05-2015, 11:37 AM
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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

2003 ford explorer v6

Wifes ride/2009 6.4 F350 SRW CCLB 4x4 135k
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post #9 of 23 (permalink) Old 06-06-2015, 09:19 PM
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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?
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post #10 of 23 (permalink) Old 06-07-2015, 07:25 AM
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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.

2013 Silverado 2500HD LTZ Z71
4x4, crew cab, short bed
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