When I 1st reset the avg MPG and I am not moving, it will read "0" MPG. Sometimes I will leave the key on when I gas up so that I can start up & move forward immediately in EV mode (without having to start the ICE). In that situation the readout will immediately go from "0" to "99" MPG.

So the question remains - should I expect the dash MPG computer (I don't have the NAV system) to read within 0.5 MPG of the actual achieved? That is acceptable - but reading 1.5 to 2.0 MPG higher than actual achieved is not acceptable!

 

You should expect +/- 2 MPG at the WORST.

Usually, it is within +/- 1.5 MPG of actual.

Mine is generous, and reads high for the most part.
But never more than +1.5 MPG of actual.

 

Just an observation, but +/- 2 mpg out of 30 is a 6 2/3 % accurate system, which isn't too tight when one thinks about it.

Thanks,
John Dixson

 

The dash econ or the nav sys is not a tank MPG average display. So far, the only tank MPG average I've used is the SG. On my nav system, I find that 200 miles is the most data it stores before dropping data. I've seen as much as a 5mpg change between the SG and the nav system. The change can be plus 5mpg or minus 5mpg depending on your driving.

If you want a real MPG tank average, get a SG.

GaryG

 

Gary, I did just that earlier this evening, for the reasons stated!

Thanks,
John Dixson

 

The problem with MPG interpretation is that many drivers do manual calculations then question the computer. In fact, it should be the other way around. Per-tank manual calculations vary tremendously because the parameters used are so inaccurate, while the dash readout (and SG) use inputs like injector flow rate, number of injector activations, etc. The readout, as previously stated, does some averaging.

As for thinking +/- 1 or 2 MPG is inaccurate, we're forgetting that it's not always +2 nor is it always -2. Besides they generally cancel each other out. If someone found theirs was "always" +2 MPG or -2 MPG then, yes, that would be less than ideal. Mine is pretty tight but I'm sure they vary from car to car. To be a fair reflection of what I'm "really" getting, I try to remember to re-set mine every 4-5 tanks.

If you re-set it every tank, it calculates the same as it did the last tank and the one before that and so on. So it will just be a snapshot of that one tank. Manual per tank calculations leave so much room for error that they are usually way off.

Fortunately, all calculations will tend toward accuracy if you take enough samples and you dilute errors across a lot of data. For most drivers, manual calculations, SG per-tank calculations and the dash readout all tend to come together over time.

 

Terry, did you read post #7 in this thread? You cannot have a 4-5 tank average with this car. Some people think you cannot have a 1 tank average.

 

Thanks, mtberman, and you are of course absolutely correct in how to best treat data that has an accuracy band around any specific set of that data.

The post number seven is interesting: I wonder if anyone has attempted to contact Ford Engineering on that issue?

Thanks,
John Dixson

 

I agree, it would be interesting to hear from Ford about how long the data is carried. The only thing we really know now is what our own, personal experience has been. Here's how mine behaves:

Initially, it appeared to be lifetime MPG since new. Late this summer I crossed the 5000 mile mark and also began driving in a pattern such that my mileage should have gotten better. My manual calculations showed a significant increase, but the display lagged. It went up, but only a few tenths over a few weeks. My own calculations showed I was probably getting 2-3 MPG MORE than the display.

I decided to re-set it to purge the data from when it was new and I was making more low MPG trips. After the reset, it went up & down but soon settled into a readout that nearly matched my own checks.

The longer I left it, the less it varied. When the weather got colder, I expected a drop. But it took weeks for the display to reflect it. Again, it slowly moved by a few tenths over a few weeks. My calculations now showed I was getting 2-3 MPG LESS than the computer displayed.

My conclusion: My experience was that relatively sudden MPG changes are reflected quickly if I recently did a reset, but seem to take forever if it's been over, say, 800 miles. That would indicate that the computer might accumulate data over many months and miles. Maybe indefinitely, i.e. counting everything since the last reset.

If you re-set it, I believe it erases everything as John says and starts over.

I'm not sure, of course, but that's what I've seen. As such, I find it to be pretty accurate as long as I reset it every couple of months.

 

You know, from what you've observed, it sounds as though the data sampling rate might change with either elapsed calendar time or mileage driven ( i.e., amount of data placed in memory ) or a blended function of the two. Someone obviously made it somewhat sophisticated in any case.

Thanks,
John Dixson