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Discussion Starter · #1 ·
What difference are you guys seeing between your hand calculated mpg and what the vehicle shows in the dash?


My 1500 used to be between .5 and .9 off of what was shown in the dash (for the worse). My 2500 has been between .7 (rarely that close) and 1.6, and seems to be getting worse for some reason lately.


Over 1 mpg off is unacceptable to me. I don't get why the truck's calculations are so far off.


Does anyone else have numbers like these and what about the mpg getting worse with the same driving habits?
 

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2015 RAM 1500 SLT 4x4
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I avg. maybe 1 mpg difference ! As for the truck PC reading nothing is perfect !
 

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What difference are you guys seeing between your hand calculated mpg and what the vehicle shows in the dash?


My 1500 used to be between .5 and .9 off of what was shown in the dash (for the worse). My 2500 has been between .7 (rarely that close) and 1.6, and seems to be getting worse for some reason lately.


Over 1 mpg off is unacceptable to me. I don't get why the truck's calculations are so far off.


Does anyone else have numbers like these and what about the mpg getting worse with the same driving habits?
The computer MPG is just a feel good reading. Mine can vary up to 2 MPG. Seldom is it within a 10th. Never does it error in my favor. All three of my rams and three town and countrys have done the same exact thing.
 

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2014 345ci 8hp70 44-44 3.92 Truetrac NX Express Comp Greene Racing
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When your fuel trims are in open loop(+40 % throttle and/or <~160 degree water temp) it’s completely estimated as there is a factory rich tune to compensate for cold dense air. Take a full tank trip and fill up when it’s warm, probably be close to the button.
 

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'18 RAM 3500 4WD Tradesman 6.4
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Ours is reading about 1 - 1.5 mpg on the optimistic side, which is about a 10% error, which I think is kinda high.



Actual mileage running empty is about 15, and with an in-bed Arctic Fox camper (makes for a total of about 3500# payload) we get about 11.
No complaints.


On our Subaru, the mileage display was off by only 5% from the factory, and the dealership can go in with their SSM (the Subaru equivalent of the RAM dealers' Witech) and dial the calibration up or down in 1% increments. Now it's real close to spot on.


I don't think the RAMs have any way to tweak the calibration though, which is a shame. It would only require a few lines of code and cost almost nothing.
 

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Discussion Starter · #6 ·
I avg. maybe 1 mpg difference ! As for the truck PC reading nothing is perfect !

I had a 2015 Chevy 1500 that would be around .5 or better and the 2012 Tundra i had before that would be spot on or a couple tenths off.

When your fuel trims are in open loop(+40 % throttle and/or <~160 degree water temp) it’s completely estimated as there is a factory rich tune to compensate for cold dense air. Take a full tank trip and fill up when it’s warm, probably be close to the button.

I calculate multiple fill ups on a long trip towing the camper and it makes no difference.

The computer MPG is just a feel good reading. Mine can vary up to 2 MPG. Seldom is it within a 10th. Never does it error in my favor. All three of my rams and three town and countrys have done the same exact thing.

Not a feel good number when i know how far off it is.


If the variance was consistent, it would be a somewhat usable number. Since it is so far off and the variance is different every time, it is totally useless.
 

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Just remember the errors from filling up, factor those into your hand calculations. Two-tenths of a gallon over several fill-ups makes a difference.
 

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Just about everything the EVIC displays (calculated MPG, Oil %, Miles to empty, outside temperature, etc...) are only marginally accurate. The NAV is worse. They're only ballpark figures.
 

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I am not sure exactly what inputs it uses to calculate MPG, but its not gallons at the pump vs miles driven. I've had mine tell me I got worse than hand calc before though it is usually the other way around. Hand calculating at the pump isn't the be all end all either as there are variables that enter into that equation too.
 

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If the variance was consistent, it would be a somewhat usable number. Since it is so far off and the variance is different every time, it is totally useless.

I agree- useless. My Mustang always errors in my favor so at least I can depend on hand calculating being better.
 

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This issue is that it is calculated on less then a full tank of gas. Roughly the last 250-300 miles. So it will almost never match hand calculated over a larger distance.
 
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