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Discussion Starter #1
The thread title may be misleading....

I am posting some observations, and theories; hoping others will join in and provide their input.

Hopefully, we can get an idea how the fuel econ function works, it's accuracy, and to what degree we should rely on it to give an accurate picture.

Observations:

Miles to empty will increase with less agressive driving (Duh!). I have observed the figure of miles to empty at the time I started a trip to increase with less agressive driving, as well as seeing it increase during the course of a trip due to consistent driving.

Consistent RPM levels, as low as possible to maintain traffic speed result in highest MPG figures

I have found that it is quite easy to accelerate to traffic speed while keeping RPM below 2000.

Idle time causes a decrease in MPG.

The display will toggle off after a set time period while vehichle is in motion. Audio Text info at top of display is not interrupted.

The Audio text info will toggle off after a set period of time if the fuel econ button is depressed while vehicle is stopped with engine running (stop light, etc.).

Measurement of Distance to Empty is not instantaneous - watch yours after a fill-up or any addition of fuel. It will go up with every cycle of the ECU's "task manager" until it reaches the actual measurement that reflects the current fuel level.

Theory:

It appears that the calculations are based on the miles to empty figure, which should be an indication of measured fuel level.

It also appears that calculations factor in accumulated run time since last reset, and don't properly "weight" the results taken when the vehicle is not in motion and the engine is running (stop light, etc.).

I would think that by placing a higher bias on calculations taken while vehicle is in motion than while it is at rest would provide more consistent and accurate figures.

If there was a "weighting" factor, then there should be a resulting ratio of motion to stop measurements that would tell the ECM to either bias calculations based on time stopped (if the total time stopped was higher than in motion), or time in motion (if time in motion is higher than time stopped).

It appears that the audio text and fuel economy functions are dependent on the motion of the car as far as which one toggles off after a set amount of time.

Fuel Econ figures measured under acceleration, braking or cornering will probably be less accurate due to the possibility of the fuel level reacting to the stance of the car.

Fuel Econ figures should fluctuate during the next few cycles of the task manager, until the actual fuel level is measured (providing the theory that the actual calculations are based on the measured fuel level). This should allow for more accurate measurements after filling with fuel and after steering, acceleration or braking that temporarily shifts the fuel level.

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I wonder if it would be possible to flowchart both observations and theories on/of operation in parallel charts until a concensus can be reached on how the system is programmed in the ECU; documenting each stage of the operation.

I learned a long time ago that when trouble shooting un-documented problems that it is easier to tell what part is failing when you know what the system looks like when performing properly, and at what stage of operation that each part works. In other words, if we can get an understanding of each step of the process in the prediction of fuel economy, we can better tell if the system is accurate, when it is most accurate and when it is less likely to be accurate.

I hope this makes some sense, and the outcome will be useful to others..
 

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Mr. 3 K, 3/3/5. 5K,10/5/7
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Discussion Starter #3
Interesting Read!..

I see it references Vehicle Speed and Air Flow as the 2 signals that it uses to compute MPG.

I know that Chrysler uses both a SAE J1850 Variable Pulse Width signal, along with Euro OBDII Stage 3 command set for advanced functions; a problem that I faced when trying to interface with the PT Cruiser turbo I traded in on my Murano.

I think the Murano is using a few more sensors in the mix to give the combination of Average MPG and Distance to Empty, while also monitoring Accumulated Run Time, Time at Rest and Time in Motion.

From what I can see in the service manual, the Unified Meter and AC Amp receives signals from both the Main and Sub Fuel Level sensors, amd mirrors input signals used by the actual Combination Meter Assy (cluster), as well as the mode and state of the CVT. It also shares data (via the CAN buss) with all the other "controllers" on the Buss.

That places a lot of data for some advanced number crunching, and it becomes a matter of programming and processing time to determine the accuracy and usability of features like the Fuel Economy.

It would be nice to see other "Trip Computer" functions added to the mix, but that is wishful thinking.....

What I would rellay like to know is what combination of driving conditions and driving style is most likely to produce the most accurate fuel economy figures via the center display.

I suspect that the EPA numbers are biased toward actual highway speed limitations and minimal RPM to maintain constant speed.

I have found the Cruise Control to be fairly good at achieving this; almost as well as driving barefoot and watching a vacuum gauge....
 

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Mr. 3 K, 3/3/5. 5K,10/5/7
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