I want to throw something out here. I just responded to a post that suggested a programmer will throw off your DIC MPG calculation, making it inaccurate. So I thought I would start a discussion about exactly how the MPG calculation is obtained by the DIC.

I don't know anything about what parameters are used for the calculation, but after years of observation with several vehicles, I do have a pretty good idea of how driving affects the reading, and I am interested in supporting/opposing viewpoints, because it seems to me that a programmer should not affect the MPG accuracy of the DIC.

Here's my take...

The DIC, when showing average MPG, is a running average with no time limiting factor, and like any average, the longer the sample, the less volatile or more stable the number. For instance, if I reset my MPG's and then immediately drive on the hiway at a steady state 60 MPH for 5 minutes, my MPG reading will be somewhere in the 18 MPG range, average. Now, after that 5 minutes, I get off the hiway and POUND the thing around town, that average reading is going to immediately drop like a rock, maybe down to 8- 10 mpg, or less. But remember, this is still an "average" reading.

Now, I reset the MPG reading again and then again immediately drive on the hiway at a steady state 60 MPH, but this time I drive across the country without stopping (must have one of those titan tanks...). I now have an average reading on my DIC of 18 MPG, but it is a MUCH DIFFERENT average than the 18 MPG average of the previous example.
Again, after that cross country drive, I get off the hiway and POUND the thing around town again, that average reading is not going to move by even a tenth this time, even after driving around town all day, because the average I previously built up is so strong from so many miles of driving.

The reason I bring this up is that if you have an accumulated average MPG in your DIC and don't reset it, the only thing it shows you is what your average MPG's were since the last reset. If you made any kind of a change, programmer, exhaust, etc., that change will not be truly reflected in MPG reading until the DIC is reset.

Many people on this site say to not rely on the DIC, but I have found it to be very reliable when checking via hand-calc. I would like to continue relying on it after I install the Stealth2

Any thoughts?

Except I've read here that the DIC mileage reading is a moving average ... over the last few hours or miles. I don't know if that is true, but it would partially defeat the logic in your second example.

More importantly, you're assuming that the DIC is comparing "fuel flow" to the odometer change (miles traveled). But there's a rumor (here) that the DIC economy readout is defeated by any programmer's ECM changes ... inferring that the economy readout depends on some "performance interpretations" within the ECM, rather than simply a fuel flow measurement. I don't know if that's true, but I'm curious.

Exactly WHICH SENSORS are feeding the DIC's economy calculations? ... does the General install a fuel flow transducer?

when you start adding fuel the dic starts to become inacurate Nick duramaxtuner made a nice tow tune tht we tested tht is dic acurate and still gives a nice increase in power i tested this tune over 1800 miles and was with in 1% of what the dic said comapred to my hand calc

Except I've read here that the DIC mileage reading is a moving average ... over the last few hours or miles. I don't know if that is true, but it would partially defeat the logic in your second example.


I also read here that the DIC is a moving average, but when you analyze it's operation over the long haul, that does not seem to be the case...

when you start adding fuel the dic starts to become inacurate Nick duramaxtuner made a nice tow tune tht we tested tht is dic acurate and still gives a nice increase in power i tested this tune over 1800 miles and was with in 1% of what the dic said comapred to my hand calc

I would hope that I get that kind of DIC accuracy with my Quad...

I'll find out tomorrow when I install and lose kitty.

Hey fire, what kind of mileage gain did you see with the exhaust & tune?

the dic is an average of when the truck is running has nothing to do with if you are moving or not. if you rest you dic and drive i don't know 5 miles than let truck sit and idle you can watch the dic drop.

last weak i drove from cheyenne wy to denver co (DIA) and back. DIC said 21.9 and hand figured was 21.78. Not bad in my book.

the dic is an average of when the truck is running has nothing to do with if you are moving or not. if you rest you dic and drive i don't know 5 miles than let truck sit and idle you can watch the dic drop.

Yes, I have observed that on a freshly reset reading. But, as in my 2nd example, if you have many miles since last reset, you can sit at idle all day and the reading won't change...

last weak i drove from cheyenne wy to denver co (DIA) and back. DIC said 21.9 and hand figured was 21.78. Not bad in my book.

And thats with your Stealth2... That's good to hear. What tune are you running?

110 hp and yes

i went from avg of 350miles on a tank to avg of 450 on a tank and have seen a max of 550 miles on long trips on a tank of fue.l the way i count my mpg is by how far i go on one tank of fuel seems to be the true test for me

the dic is an average of when the truck is running has nothing to do with if you are moving or not. if you rest you dic and drive i don't know 5 miles than let truck sit and idle you can watch the dic drop.

"Moving average" doesn't mean it computes only when the truck is moving.

A moving average means the computer is constantly deleting earlier results in favor of more recent results. As an example, the computer uses only the most recent 2 hours (or 1 hour, or 3 hours) of data, and ignores older data.

I only describe this as what rumors here have inferred ... I don't believe it, since my DIC reading always seems very close to hand calcs. Besides, it would be an unnecessary complication for the General to save that block of consumption data and continually update it. Why would they do that~!? ... doesn't make sense. I'm betting the DIC economy is simply a readout of cumulative miles divided by cumulative fuel flow ... a simple average since the last reset.

I still see no evidence that a Programmer will alter the factory's economy readings. There MUST BE a fuel flow transducer somewhere, right? My '03 Sierra shows economy on the dashboard too, and it's always been very accurate. Both my '03 and my 07+ also show "gallons used" since last reset. The only difference I can fathom is that some diesels recirculate unused fuel back to the tank ... is that true for the D'max??

Does anyone know the real answer of how or why a Programmer changes the economy calculation??
Or is that just a confused rumor?

Even with the stock tuning the DIC is just an estimate based on things like TPS, Speed, RPM, Boost, Manifold Pressure, and it isn’t read from the ECM. The cluster has formulas for these things and it uses those formulas to determine a fuel mileage estimate. What I’m saying is that the DIC isn’t communicating with the ECM to do fuel over mileage calculations, because if that was the case the DIC wouldn’t get thrown out of whack. But since it uses formulas that are based on a stock engine with a stock tune; things get whacky when you increase things like fuel and boost and there are different levels of power at different RPMs.

To say it simply, the parameters that the DIC uses to calculate fuel usage, (pulse width and duration) are changed by some tuners in a way that the DIC does not recognize, thus more fuel used than DIC calculates. Correct?

To say it simply, the parameters that the DIC uses to calculate fuel usage, (pulse width and duration) are changed by some tuners in a way that the DIC does not recognize, thus more fuel used than DIC calculates. Correct?

That’s not exactly true. Because we alter the fueling quantities and timing with the same tables the factory does so the PCM knows exactly how much fuel is entering the engine (unlike an in-line module which alters after the PCM). But, because the DIC bases its fuel mileage estimates off of a theoretical model of the factory engine with a factory tune, the tuning changes are not accounted for (even when using the factory tables with our changes). If the fuel mileage was being directly calculated directly from PCM fueling quantities and vehicle speed/distance traveled then our changes would not induce errors. Because our tuning does change the engine fueling at a given TPS/Boost/RPM/etc. that no longer matches the DIC model, thus causing an error between actual and calculated fuel mileage.

Also, realize that while the DIC estimate is typically very close with a factory tune, it is rarely exactly correct to what you may calculate at the pump. This shouldn’t be the case if it truly calculates the known fuel quantity entering the engine.

That’s not exactly true. Because we alter the fueling quantities and timing with the same tables the factory does so the PCM knows exactly how much fuel is entering the engine (unlike an in-line module which alters after the PCM). But, because the DIC bases its fuel mileage estimates off of a theoretical model of the factory engine with a factory tune, the tuning changes are not accounted for (even when using the factory tables with our changes). If the fuel mileage was being directly calculated directly from PCM fueling quantities and vehicle speed/distance traveled then our changes would not induce errors. Because our tuning does change the engine fueling at a given TPS/Boost/RPM/etc. that no longer matches the DIC model, thus causing an error between actual and calculated fuel mileage.

Also, realize that while the DIC estimate is typically very close with a factory tune, it is rarely exactly correct to what you may calculate at the pump. This shouldn’t be the case if it truly calculates the known fuel quantity entering the engine.

I stand corrected. The "tuners" I was refering to should have been called a "fuel module" as in, Edge Juice is what i was thinking of. The tuners that change the program itself should not have as much effect on the DIC.
My bad!:o:

I just installed the Stealth 50 HP tune, so I'm anxious to do some hand calcs and see how the DIC reading holds up

hahahah hmmm well try running a 500 hp tune or so and see wht the dic reads mine says im gettin 30 mpg at times and avg of 25mpg i wish just not true. i do however avg about 19 mpg hand calc