EVIC fuel economy - how does the ECM calculate it?

[m35a2]

I've seen a few threads regarding mileage calculation and how the EVIC can be inaccurate.
I've noticed about a 2MPG difference between EVIC's mileage and my own hand calculation. However, I question the accuracy of the number of gallons I'm using. A couple of gallons difference can make a significant impact. In my old F250 automatic shutoff could and often did stop pumping fuel well before the tank was full, seemingly intermittently. Could the same thing be happening when I fill up my new Ram?

So my question is, "How does the EVIC calculate mileage?"
It seems like the ECM should know the fuel rail pressure, how long the injectors are on, how much fuel an injector flows when "on", and thus have the ability to calculate how much fuel is flowing.

I wonder which is more likely to be wrong - the ECM or my sense that the tank is at the same fullness level as the previous fillup?

Discuss!

 

[Lynx0849]

I've seen a few threads regarding mileage calculation and how the EVIC can be inaccurate.
I've noticed about a 2MPG difference between EVIC's mileage and my own hand calculation. However, I question the accuracy of the number of gallons I'm using. A couple of gallons difference can make a significant impact. In my old F250 automatic shutoff could and often did stop pumping fuel well before the tank was full, seemingly intermittently. Could the same thing be happening when I fill up my new Ram?

So my question is, "How does the EVIC calculate mileage?"
It seems like the ECM should know the fuel rail pressure, how long the injectors are on, how much fuel an injector flows when "on", and thus have the ability to calculate how much fuel is flowing.

I wonder which is more likely to be wrong - the ECM or my sense that the tank is at the same fullness level as the previous fillup?

Discuss!

I watched a video of guys out in Colorado doing fuel mileage tests of pickups.

For accuracy they followed a strict procedure.
They always filled at the same pump even if they had to wait.
They filled till it shut off. They then waited 30seconds and filled again till shutoff. No trying to get a little more in.

 

[brulaz]

Yep, I figure most of the random error is due to the pump/fill process.
But beyond that, there can definitely be a systematic error, with the EVIC about 4% optimistic in my case, .

 

[p-Bar]

I watched a video of guys out in Colorado doing fuel mileage tests of pickups.

For accuracy they followed a strict procedure.
They always filled at the same pump even if they had to wait.
They filled till it shut off. They then waited 30seconds and filled again till shutoff. No trying to get a little more in.

that is a skewed procedure for fill up . Their is a lot of foam that builds up as it fills so the more empty the tank is the more foam that will build up. The only way to do it is top it off till you see liquid. Sounds like the EVIC is working better than on my 04.5 Lie-o-meter.

 

[jhenderson]

4% is nothing. Mines been off as much as 2.5 mpg. If memory serves me it's only been right 3 times in almost 60K. The ECM uses an algorithm comprising throttle position,map,rpm,And miles driven, among others. Contrary to popular belief, the system has no fuel totalizer.

 

[brulaz]

4% is nothing. Mines been off as much as 2.5 mpg. If memory serves me it's only been right 3 times in almost 60K. The ECM uses an algorithm comprising throttle position,map,rpm,And miles driven, among others. Contrary to popular belief, the system has no fuel totalizer.

4% was the average difference. Individual fill-ups have been a lot worse, both pos and neg deviations.

 

[sag2]

4% is nothing. Mines been off as much as 2.5 mpg. If memory serves me it's only been right 3 times in almost 60K. The ECM uses an algorithm comprising throttle position,map,rpm,And miles driven, among others. Contrary to popular belief, the system has no fuel totalizer.

Actually it does know approximately how many gallons are consumed. That is how it calculates. It works just like m35a2 thought it did. It knows pressure, time the injector is commanded open, and the theoretical flow of the injector. It keeps track of all the data and calculates how many gallons are consumed. My neighbors 06 was off several MPG all the time. After we put a set of injectors in it is much more accurate.
My Hemi is within 1.5 MPG unloaded, and about .5 MPG when towing. My wifes 200 is almost right on. They are always on the optimistic side, something I would expect from any good marketing guys.

 

[Cummins12V98]

Miles driven divided by gallons used equals miles per gallon. The EVIC is just a toy. Mine is consistently .4 mpg high.

 

[jhenderson]

It' still an algorithm. There is no mechanical measurement made. That's why it's not dependable. Unlike brulaz, mine has never told me my mileage was worse than actual. When you're talking 2.5 mpg at an actual 18 that's working on a 15% deviation. That's not a reasonable variance if the truck was actually measuring fuel use. Your neighbor may have had mechanical problems but I don't. We're talking a new truck with less than 60K on it. This is also my experience on my 04 for a total mileage of 385,000 with every gallon tracked.

 

[mwilson]

Mine has been consistently high by 2 mpg, sometimes more since the day I bought it new in '06.

Hoopty on the other hand is dead nuts on every time. I use the "Fuelly" software on my phone to track fuel mileage and maintenance records. It matches almost exactly every fill up..hoping RAM at some point might have hired the GM guy that wrote the Cadillac algorithm for both fuel mileage and oil change intervals in '06.....

 

[jhenderson]

Do you think they'd get a package deal if they hired the HVAC engineer too? Maybe the bottom 4 inches and wipers would deice along with the rest of my windshield if they aimed the defroster correctly. I thought they tested these things in winter before they went into production.

 

[Jim W]

You are asking for a lot! Especially since the engineers do not have a say in the styling compartment of any vehicle that is designed today, most stylist do not believe in FIT, FORM, FUCTION. This was my experience when I was design tractors, we always had issues with the stylist.

 

[TCDiesel]

4% is nothing. Mines been off as much as 2.5 mpg. If memory serves me it's only been right 3 times in almost 60K. The ECM uses an algorithm comprising throttle position,map,rpm,And miles driven, among others. Contrary to popular belief, the system has no fuel totalizer.

I'm with you on this one..The FCA cycle and Voltage Vs injector Duration Pressure would consume bandwidth that's Not available at present. We have install bigger injectors that require lower Voltage to FCA and lower cycle the LOM reads the about the same MPG even at consumption considerably higher. I am surprised at how accurate most LOMs +/ - % are thou.

 

[spoon059]

The EVIC is just a toy. Mine is consistently .4 mpg high.

I usually use the local Freestate (cash only, high volume dealer) and they have 2 dedicated truck pumps and a 5 cents/gallon discount if you buy 25+ gallons. I use the same pump every time (unless I am on a trip) and fill until it shuts off, wait, then fill again. I find that my EVIC is usually a half gallon optimistic, which is good enough for me. I don't rely on the EVIC to be exact, but I like to have a reasonable estimation of current conditions and driving manners. For that purpose, I find the EVIC to be great tool.

 

[Jared Herzog]

My truck is always off from 2-5 mpg. At this point i am sure it is just guessing.

 

[brulaz]

Dunno what happened early on or at the last fillup. Prolly a weird pump or a I misread the numbers.

 

[Badunit]

My EVIC is averages 1.2 mpg above reality. It has been as much as 3 mpg high and as much as 1 mpg low (though that was only one time, was in the first 5k miles, and was only an 11 gal "refill"). Typically it jumps around between 0.5 to 1.5 mpg high. I don't try too hard to fill to the exact same spot each time because it is going to be inaccurate no matter how hard I try.

Whatever the EVIC algorithm is, it is based on a lot of small "measurements". It like using a 1 ft ruler to measure a mile, one foot at a time. You may end up in the ballpark but it won't be very accurate. And, with the added incentive to meet customer expectations, the EVIC is obviously skewed to the high side vs trying to be as accurate as possible.

 

[jhenderson]

Very good analogy.

 

[Flash1034]

I will say..I'm surprised. My evic "average fuel economy" number is usually within 1 mpg of my hand calculation...