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  #1  
Old 12-14-2010, 06:48 AM
leon leon is offline
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Default Fuel Economy Display Inaccurate

I'm a new owner of a 2004 Ram 2500 5.9L automatic 2wd. I believe the truck to have the stock ECM tune in it. I'm getting about 20 mpg, but the onboard computer reads 24.5.

I haven't reset it for about 2000 miles, so I figure its showing what it thinks is the long term average. Is it normal for it to be this optimistic? Is there any way to tweak it so it reads closer to reality? I figure the calculation is based on the amount of time the injectors are open, so does the disparity mean I have a fuel leak somewhere?

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Old 12-14-2010, 07:57 AM
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Has your tire size or gearing changed from stock? That would make a difference. Is your speedometer accurate?
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Old 12-14-2010, 08:12 AM
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It's adaptive. If you watch it for a minute or two or three while idling vs. driving around, it will change. It is not the most accurate gauge I have seen for sure. I just get my MPG by doing the math.
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Old 12-14-2010, 08:26 AM
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It's not meant to be super accurate but rather a ballpark figure.
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Old 12-14-2010, 09:49 AM
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Yup, it's never quite right, nothing beats the good ol' miles driven/gallons filled.
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Old 12-15-2010, 07:23 AM
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My speedo is right as compared to a GPS. I don't expect the onboard computer to be exact, but like RamTech says, a ball park figure. But its 20% off. That's what got me to thinking something might be wrong. Assuming the onboard computer is using the amount of time the injectors are open to calculate fuel usage, and since I'm using the amount of fuel I put in the tank, the difference indicates missing fuel. There's about 18 gallons of fuel unaccounted for. I don't see a leak that big. That's a little over an ounce per mile.

Does anyone know where there is technical documentation on the on board computer?
The service manual isn't very informative.

regards,
Leon
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Old 01-06-2011, 10:59 AM
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Mines off about that much or more. That thing is a waste of a feature. Either be right or don't put it on the truck.
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Old 01-06-2011, 11:26 AM
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only time i know when im getting good gas mileage is when im trying to conserve it on the highway.. other than that.. my local driving mileage is horrible with all the starts and stops with stop signs.
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Old 01-06-2011, 02:15 PM
Dalton06 Dalton06 is offline
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Yeah it's so uphill and downhill in ga it's hard to get good true reading.
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Last edited by Dalton06; 01-06-2011 at 07:19 PM. Reason: Spelling
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Old 01-06-2011, 06:47 PM
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Quote:
Originally Posted by Dalton06 View Post
Mines off about that much or more. That thing is a waste of a feature. Either be right or don't put it on the truck.
Perfectly put. I lose 4 mpg if/when I go around corners and this truck is new=huh
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