Interesting fuel mileage with 6.7

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68PowerWagon

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First diesel I have ever owned. I know most people don't trust what the dash tells you, but my past vehicles have never been off by more than 1mpg from hand calculations. Anyway mine will be showing 20mpg rolling down the highway for maybe a good 20 or 30 miles then will drop pretty fast to about 18mpg. with all of the same road conditions. Does this erratic reading have anything to do with the DEF? Maybe injecting more or less at one given time over the other? I have yet to do a hand cal. on it, as I have only had it about a month. And no, I am not complaining about the mileage... this is GREAT compared to my 2016 6.4 gasser I had.
 

John Jensen

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DEF has no effect on MPG. An active regen can show a reduced MPG on the current reading because fuel is being injected into the exhaust system to raise the exhaust temps.

Are you watching the current or average MPG numbers?
The current will act as you describe, I like the 99mpg when coasting on a downhill. The average will be close to the hand calculated data.
 

huntergreen

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First diesel I have ever owned. I know most people don't trust what the dash tells you, but my past vehicles have never been off by more than 1mpg from hand calculations. Anyway mine will be showing 20mpg rolling down the highway for maybe a good 20 or 30 miles then will drop pretty fast to about 18mpg. with all of the same road conditions. Does this erratic reading have anything to do with the DEF? Maybe injecting more or less at one given time over the other? I have yet to do a hand cal. on it, as I have only had it about a month. And no, I am not complaining about the mileage... this is GREAT compared to my 2016 6.4 gasser I had.
Your fuel mileage will drop during a regen.
 

tjfdesmo

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Someone on TDR figured out the MPG average is not a true running average, but resets about every 200(?) miles. It will jump around. My '19 Lie-O-Meter would consistently read 1.5+ MPG optimistic versus hand calculated. Interestingly, our Grand Cherokee's readout is much more accurate, usually <1 MPG difference.
 

John Jensen

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Someone on TDR figured out the MPG average is not a true running average, but resets about every 200(?) miles. It will jump around.
I've never heard that, it's hard to believe.

I've done 600+ mile stretches and the average has never "reset". Nor has it ever reset between a fillup and the next fillup.

It has never "jumped" around.

"is not a true running average"? It always comes within 1-2 MPG of my hand calculated average.

The above is how both my 2016 Eco and my 2020 HO act.
 
OP
OP
68PowerWagon

68PowerWagon

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DEF has no effect on MPG. An active regen can show a reduced MPG on the current reading because fuel is being injected into the exhaust system to raise the exhaust temps.

Are you watching the current or average MPG numbers?
The current will act as you describe, I like the 99mpg when coasting on a downhill. The average will be close to the hand calculated data.
No I am looking at the average.
 

06 Dodge

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First diesel I have ever owned. I know most people don't trust what the dash tells you, but my past vehicles have never been off by more than 1mpg from hand calculations. Anyway mine will be showing 20mpg rolling down the highway for maybe a good 20 or 30 miles then will drop pretty fast to about 18mpg. with all of the same road conditions. Does this erratic reading have anything to do with the DEF? Maybe injecting more or less at one given time over the other? I have yet to do a hand cal. on it, as I have only had it about a month. And no, I am not complaining about the mileage... this is GREAT compared to my 2016 6.4 gasser I had.
I have noticed on my 2022 that the DTE shown on the dash or when looking at mpg average will drop when in Regen, as far as hand calculating my mpg numbers depends on my right foot, my mpg numbers can be as far off as 2 mpg or be within 1/2 mpg of what's listed on the dash so I when I look at the average mpg number given just subtract 2 mpg for a ballpark number...
 

JayLeonard

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Someone on TDR figured out the MPG average is not a true running average, but resets about every 200(?) miles. It will jump around. My '19 Lie-O-Meter would consistently read 1.5+ MPG optimistic versus hand calculated. Interestingly, our Grand Cherokee's readout is much more accurate, usually <1 MPG difference.
My lie o meter is about the same close to 2 mpg better than hand calculated.
I read somewhere that its 300 miles running average.
 

BossHogg

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First diesel I have ever owned. I know most people don't trust what the dash tells you
Wow, first diesel, and it's a 6.7L Cummins, awesome.

Many do not understand how the average MPG on the display is calculated. To mitigate this, some manufacturers have added a sub-display to show the average MPG over the last 50 miles and/or the last 100 miles. The computer knows exactly how much fuel is injected into the engine, there is little room for error. The average MPG is calculated from the instant MPG and so you can see why "over time" is important to the accuracy of the average MPG.

The average MPG is just that, the average MPG over time. The longer the average is calculated, the more accurate it becomes. A few years ago I did a one-year experiment. I hand-calculated at every fill-up and did not reset the average MPG display. At the end of one year, the difference between hand calculated and the DIC's average MPG was a few 10s likely attributed to my inability to refill the tank to the exact level of previous refills.

Hand calculations are likely the most inaccurate because hand calculations do not consider fuel expansion and contraction based on temperature. There is also no method that guarantees you fill the tank to the same level each and every time. For example, a mere 3/10s of a gallon delta in a refill can equate to an error of 0.45 MPG with respect to miles traveled and the amount of fuel used.

Now there are a lot of folks that disagree with me but I can tell you, over my automotive career, I worked on every vehicle system except ABS and SRS (safety restraints) and I have first-hand knowledge of how several programs I participate in, and that is how it was done. If you record your hand calculations and are honest about it, you would see a variance in your MPG for each fill-up. I did.
 

KKBB

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Wow, first diesel, and it's a 6.7L Cummins, awesome.

Many do not understand how the average MPG on the display is calculated. To mitigate this, some manufacturers have added a sub-display to show the average MPG over the last 50 miles and/or the last 100 miles. The computer knows exactly how much fuel is injected into the engine, there is little room for error. The average MPG is calculated from the instant MPG and so you can see why "over time" is important to the accuracy of the average MPG.

The average MPG is just that, the average MPG over time. The longer the average is calculated, the more accurate it becomes. A few years ago I did a one-year experiment. I hand-calculated at every fill-up and did not reset the average MPG display. At the end of one year, the difference between hand calculated and the DIC's average MPG was a few 10s likely attributed to my inability to refill the tank to the exact level of previous refills.

Hand calculations are likely the most inaccurate because hand calculations do not consider fuel expansion and contraction based on temperature. There is also no method that guarantees you fill the tank to the same level each and every time. For example, a mere 3/10s of a gallon delta in a refill can equate to an error of 0.45 MPG with respect to miles traveled and the amount of fuel used.

Now there are a lot of folks that disagree with me but I can tell you, over my automotive career, I worked on every vehicle system except ABS and SRS (safety restraints) and I have first-hand knowledge of how several programs I participate in, and that is how it was done. If you record your hand calculations and are honest about it, you would see a variance in your MPG for each fill-up. I did.
I just did the math. If a Cummins put 400 miles on and took 30 gallons to fill, it got 13.3333 mpg. If it was 3/10's more like you say, that is 13.2 mpg. A difference of .1 mpg. Not much difference at all.
 

KKBB

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If you do the 400 miles by 20 gallons of fuel, that is 20 mpg. If you add the .3 gallons like you said, that is 19.7, so you are correct on that...and I would agree that an empty Cummins would have no issue getting 20 mpg.
 

jejb

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I just did the math. If a Cummins put 400 miles on and took 30 gallons to fill, it got 13.3333 mpg. If it was 3/10's more like you say, that is 13.2 mpg. A difference of .1 mpg. Not much difference at all.
Agreed. And given that, if the EVIC is off by 1-2mpg, as it often is on my Cummins trucks, hand calculating would be much more accurate.
 

Dusty

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There is no designed-in or arbitrary cutoff of the system Average reading. The Average is determined solely by manual resets directed by the operator. As BossHogg has stated, longterm system average readings will be pretty close to hand calculated average readings.

In reality, inaccuracies in the system readings start at the fuel station's pump when filling. Differences in the pump handle's back pressure valve and the level of the vehicle contribute to differences of how much fuel fills the vehicle tank.

I've hand calculated each time I fill since I've had the vehicle and compare system figure to the hand calculated figure. Sometimes it varies quite a bit ( + or - ) because of the fuel pump's differences. However, my to-date system average and hand calculated average are only 0.35% difference (meaning I've used .35% more fuel than the system thinks I've used).

This is my third Ram 1500 and each has so far has been within 1% or less of hand calculated.

Regards,
Dusty
2019 Ram 1500 Billet Silver Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33-gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 3 June 2018. Now at 85180 miles
 

RamRigger

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I think we get what we get. It is an imperfect system. That metering gauge showing mpg, is an estimation according to all factors involved; road condition, weather conditions including rain, snow, wind, uphill, downhill, duration of all of these conditions, longer, shorter, weight on board, drafting behind forward vehicles, passing, constant speeds vs irratic speeds and lane changing, braking, acceleration, moisture in the fuel, air/fuel ratio, fuel injection pump effectiveness during calculation, glow plug constancy, fuel injector effectiveness, and just about anything else that is among the calculating metrics. Every single nuanced input is being constantly revalued by the microprocessors involved.

I simply live with what is given to me as the systems best possible averaging outcome.

As for dropping from 20mpg to 18mpg, I believe the answer is most closely interpreted as John Jensen mentioned above

"Are you watching the current or average MPG numbers?
The current will act as you describe, I like the 99mpg when coasting on a downhill. The average will be close to the hand calculated data."
 

ppine

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Going from 20 mpg to 18 mpg is only a difference of 10%. That can be easily explained by hills, stoplights, traffic and plenty of other things. I am finding that cold winter weather decreases tire pressure which affects mileage.
 

Timsdually

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I trust the computer which is metering exactly what's going in the engine rather than guessing how much I am filling/not filling when I "think" I just filled up at the station.
 

Mikeyram

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I think when you hit reset it starts all over. Which is a good thing to discriminate different driving types such as on a 55 mile per hour highway verses a sixty fife or seventy mph road. Mine is 18 at seventy and 21 at 55. Fun thing to do
 

jejb

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I trust the computer which is metering exactly what's going in the engine rather than guessing how much I am filling/not filling when I "think" I just filled up at the station.
Not me. I take my time and fill up to the tippy-top each time I fill. It's almost always much more accurate than the lie-o-meter on the dash. On long trips, I've consistently gotten the same mpg fill after fill, driving on flat midwestern highways, while the EVIC indicated 2mpg higher. I take the EVIC mpg reading as just a very rough indicator. The only time it is close is when I'm hauling my bass boat over long distances. I reset the trip computer at each fill.
 
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