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Old 10-27-2014, 06:05 PM   #1
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Dodge MPG display 7 MPG off

Not sure where to post this question but my dad has a 2006 dodge 2500 2wd megacab 5.9 Cummins. His overhead display on a 500 mile trip all hwy gave him 24.5mpg but when he hand calculated it he actually got 17.5. That's a HUGE discrepancy in my opinion. My wife's is usually only 1mpg off so I figure his to be 1-2 off as well. Anyone have any reason for this? He reset it when he left the gas station and it did fluctuate a few mpg here and there but at fill up it said 24.5. Also isn't 17.5 kind of low? Its completely stock in every way with 150k and maybe 600lbs in the bed. We were running between 70-80 in flat south Texas. On I35
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Old 10-27-2014, 06:09 PM   #2
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Quote:
Originally Posted by dcarver View Post
Not sure where to post this question but my dad has a 2006 dodge 2500 2wd megacab 5.9 Cummins. His overhead display on a 500 mile trip all hwy gave him 24.5mpg but when he hand calculated it he actually got 17.5. That's a HUGE discrepancy in my opinion. My wife's is usually only 1mpg off so I figure his to be 1-2 off as well. Anyone have any reason for this? He reset it when he left the gas station and it did fluctuate a few mpg here and there but at fill up it said 24.5. Also isn't 17.5 kind of low? Its completely stock in every way with 150k and maybe 600lbs in the bed. We were running between 70-80 in flat south Texas. On I35
I have the exact same truck (2007). I get 20 mph empty at ~70 mph and the display usually returns 21 ish. Just put an MBRP exhaust on it and bumped up the mileage 5%. Now I'm solidly in the 21s.
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Old 10-27-2014, 08:40 PM   #3
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Quote:
Originally Posted by dcarver View Post
Not sure where to post this question but my dad has a 2006 dodge 2500 2wd megacab 5.9 Cummins. His overhead display on a 500 mile trip all hwy gave him 24.5mpg but when he hand calculated it he actually got 17.5. That's a HUGE discrepancy in my opinion. My wife's is usually only 1mpg off so I figure his to be 1-2 off as well. Anyone have any reason for this? He reset it when he left the gas station and it did fluctuate a few mpg here and there but at fill up it said 24.5. Also isn't 17.5 kind of low? Its completely stock in every way with 150k and maybe 600lbs in the bed. We were running between 70-80 in flat south Texas. On I35
Does he have a plug and play tuner installed on it? Those are notorious for screwing the mpg gauge up to make you think you're getting better mpg. If no tuner installed you can take it back to dealer and they can adjust the computer for more accuracy.

Chad
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Old 10-27-2014, 08:43 PM   #4
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Thats about right at 17-18. Depending on tires, tail wind, imput of throttle while passing etc, that what I get.
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Old 10-27-2014, 08:56 PM   #5
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No tuner and no tuning (that we know of anyway). What's weird is that he gets 13.5mpg when pulling his 10k 5th wheel 65mph. He's not complaining since his 6.0 chevy got 10hwy and 6 towing lol. Just weird that the display is crazy inaccurate. I'll tell him about the dealer reset but I doubt he'll want to pay $80 for 5min worth of work lol. I think he's a little pissed though because my 8.1 yukon xl got 14mpg on the same trip
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Old 10-28-2014, 07:46 AM   #6
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That's the kind of stuff you take to the dealer to have fixed if you are really interested.
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Old 10-28-2014, 08:53 AM   #7
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Guys, the on board mpg calculation is constantly variable. I am not sure how often it completely refreshes, but it's at least twice per tank. If the last 200 miles of that tank were highway and the first 300 were towing, both numbers are correct.
It is as if you manually reset the calculation ten miles before refueling. This would not be accurate either.
Think of the on board calculation as being "current conditions" or a short term average, not truly long term.

Mine was at 21mpg and I went and got my tt from storage. By the time I got home after 10 miles it was at 18mpg. This does not mean my average for the tank is 18, but somewhere in between.
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Old 11-02-2014, 01:14 PM   #8
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Mine in just about brand new and is about 2 mpg "happy"... That sells trucks but for those that can actually do math... not necessarily useful...
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Old 11-02-2014, 01:30 PM   #9
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They are called "lie a meter" for a reason. Glad mine don't have it.
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Old 11-02-2014, 01:43 PM   #10
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All of the information presented is based on gathered data. This seems to be a quite simple application. It would use the odometer and a flow meter. If either of the inputs are correct the output will have issues.

I have a motorcycle where the speedometer is out by about 10% high. My friend who rides a different brand is about the same. We laughingly call them H Miles. Using their information our gas mileage is quite good when you calculate it - somewhere in the 50 mpg range. If you apply the correction it is less impressive.
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Old 11-02-2014, 07:30 PM   #11
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I wouldn;t be concerned over just one tank for anything.
I reset every 8-10 tanks so I can get a good average. If after checking several tanks the overhead is still that much off have him check with a Dodge dealer for a possible flash.

My Dodge/Cummins overhead mpg is 1 to 1 1/2 mpg less than pencil figures. I've never tried resetting for just one tank.
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Old 11-03-2014, 04:24 AM   #12
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I use ScangaugeII II and it needs calibration to be accurate. Been using it 7 years and love it. It can be reset at any time without affecting tank to tank calibration. I still cannot figure why. $60k trucks cannot have it by now as standard equipment. Manufacturers just don't want to tell us anything. Afraid we could find the truth. Also discovered that some fueling stations or companies don't give you the fuel you pay for. Less often in the state but here in Canada I only fuel at Esso.
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Old 11-08-2014, 04:25 PM   #13
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My 12 year old SuperDuty says I'm getting 24.5 mpg when I'm getting maybe 18 mpg around town.

Such gauges don't work right forever--if they ever did. They're just relative things.

But a dealer would charge a minimum $100 to look at any electrical problem. It's not worth fixing to me.
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Old 11-09-2014, 06:07 PM   #14
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I use my programer and the mpg increases by up to 43%. The higher the setup the greater the difference. So I calibrate my ScangaugeII at every setup change which I do in the fall and spring for my south trip.
But setup on stock its dead on.
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