On-screen MPG vs. calculated
 

Hi folks, I've been driving my '06 FEH for a few months now, and something I'm noticing is that the on-screen MPG indicator is nearly always higher than the actual.

My latest tank *looked* great - 34.6 MPG on the display, but I was rather disappointed to see a calculated 31.9 MPG when I did the math based on mileage and actual gallons. This difference has always been there, but for the latest tank (and my current one also) it appears to be even larger.

I'm starting to think I should enter my mileage data based on on-screen MPG instead of the actual mileage/gallons - I'd be further up the list. ;)

Does anyone have any hypotheses on why there's a difference between the on-screen and calculated MPG?

Re: On-screen MPG vs. calculated
 

We have an '05 and see the same differences with the on-board computer vs hand calculation for mpg. Although there are times the computer is higher, most of the time it's been lower. Generally it's less than a 1/2mpg, but there are times it's as much as 2 mpg (either way).

Re: On-screen MPG vs. calculated
 

That issue came up previously, at least for those that have the nav system - and I think it has to do with the fact that the computer only averages out 2000 data points when calculating the average MPG - and this may be well under one gas tank. For those that have the in-dash economy display, I'm not sure if this is the same situation, but could explain why the reading is off. So instead of averaging the whole tank it only averages the last 2000 readings. Someone asked this question at the Dearborn meeting - but what wasn't clear how often those data points are collected. I think it may be every time the instant economy reading is updated.. maybe once every couple of seconds or so.

I'd be interested to know how the "miles to empty" is calculated. I filled up recently and it told me "478 miles to empty." After driving around for about 20 miles and making a one hour stop later that day I got back in the car and it said "490 miles to empty." Wonder how often that gets recalculated.

Re: On-screen MPG vs. calculated
 

I was wondering that myself.... I used my calculated amount in the database, although I would LOVE to use the display MPG!! ;) I was pretty much at 36 MPG the whole first tank, although someone had mentioned that at 0 MTE they still had 1.5 gallons left. So I set up a spreadsheet, adding my current trip reading to the miles to empty, then dividing by 13.5. At the end of my first tank, my estimated MPG was very close to my calculation after filling up, 33.5 to 33.1, and that might have been because they topped it off.

Fred

Re: On-screen MPG vs. calculated
 

Does anyone have any hypotheses on why there's a difference
between the on-screen and calculated MPG?

Several possibilities come to mind.

1) Applied computer technology is not necessarily precise - some is and some isn't. I am amazed at how well the computer technology in the FEH works , but have limited belief in its precision. (I am a computer scientist and supposedly know what I am talking about.)

2) Apparently the computer makes some assumptions about each tank - the primary one is that all full tanks are the same. They aren't, of course, as the manual points out several times. Even a few tenths of a gallon can make a significant difference in the calculations. The manual recommends determining the message center MPG on the basis of several tankfuls which should even out the variation in tanks.

3) During the Dearborn "parade," I asked my engineer (possessive, aren't I) about the basis on which the display MPG was calculated. I got the figure 2000. I thought I heard him say "miles." This would be consistent if the computer updated the estimate every mile. In any case, to the degree that 2000 data points is considerably more than a single tank, the average includes previous tanks.

I might say that point 3) is consistent with my own experience. When I took delivery of my 06 FEH 2WD, about 1000 of the first 1500 miles were highway. (Not the way I would have done it if I had had a choice!) When I started driving in city environments, the display MPG climbed slowly from 31.7 to its current 36.8 at 3800+ miles. Calculating MPG separately gives me 40.2 and 40.7 respectively for my last two tanks. So my computer MPG is lower than my actual, which differs from the other reports here.

Can the computer be set for a base other than 2000 data points? My engineer said no. It can be resect to factory default, of course. This was offered to those at the Dearborn event. I declined.
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06 FEH 2WD "Sainte Marie"

Re: On-screen MPG vs. calculated
 

This has been knocked about on each cars' forum.

Bottom line: the dash display (does FEH have one?) and your manual calculation use the odometer for distance. The navi uses GPS. Odometer is actual distance travelled (assuming its calibrated correctly); GPS is interpolated from samples over time, and the dots are connected by the computer...with varying results. Any dropouts in satellite signal, and it has to guess or dead reckon.

Each position fix can be off by up to 10 meters...compound that, especially if you're changing direction often. In reality almost all of those errors balance out. But not always, as you've found.

I've also read that some map+GPS navi systems use the calculated distance of the roads you drive on. That distance is literally measured down the middle of the road, which you are not on, especially as you turn. That difference could add up, depending on what routes you drive.

Personally, I prefer the odometer-based display. It correlates more closely with my calculated mpg, even if its not as rosy of a picture.

Re: On-screen MPG vs. calculated
 

Response to gonavy:

Don't got no navi - so all calcs are base on odometer.
Do got a dash MPG display (which displays other things too).
Dash display seems to lag manually computed MPG.
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06 FEH 2WD "Sainte Marie"