Computer MPG Inaccuracies?

<div class='quotetop'>QUOTE(pkhoury @ Jun 17 2007, 04:23 PM) [snapback]463352[/snapback]</div>

Japanese Prius drivers are saying the display mileage number is about 1% to 4% better than actual and the number is not related to the HV battery level.
So, it shows +2.5% +/- 1.5%, and I think it is reasonably accurate.

I believe it measures the fuel injector ON period for fuel usage and counts VSS or vehicle speed sensor pulse for distance traveled.
My SuperMID M-1 mileage computer does the same thing and shows very good results.

Ken@Japan

 

On a related note... what is going on in the Prius HSD, when I'm cruising along on level ground at 48-49mph, I view the MFD and see orange ICE arrows to the wheels only---yet instantaneous MPG is steady at 99.9, for about a full minute until I hit a little incline in the road?
Is it just a whisper of gas being used, such that it doesn't register enough to count as consumption?

 

<div class='quotetop'>QUOTE(ChristoB @ Jul 11 2007, 11:08 AM) [snapback]476806[/snapback]</div>

The car is moving but you are not consuming gas, so how many MPG are you getting? Zero? Wrong. N miles / 0 gallons is not 0 MPG, but rather infinity. 99.9 MPG is the Prius's representation of infinity.

Now, if you are consuming gas but not moving, you have 0 miles / n gallons, which equals 0 MPG.

 

Hi Marlin... I'm not confused about 99.9 representing infinity or division by zero. Makes perfect sense to me, to see 99.9 in a no-arrows glide, or battery-only drive...
Rather, why do I see 99.9mpg AND have orange ICE arrows from Engine to wheels, if I'm not using gas as you suggest? My understanding was that those orange arrows meant fuel was being consumed (vs. ICE spinning but not consuming---such as over 42mph, but no orange arrows showing)?

 

<div class='quotetop'>QUOTE(ChristoB @ Jul 11 2007, 11:25 AM) [snapback]476823[/snapback]</div>

Sorry, I misread your question. I thought it was the rather common question of "Why does it say 99.9 MPG when I'm not using any gas?" that we get here often.

 

I've owned 2 Prii, my 2005 consistently showed about %5 better MPG that I was getting, my 2007 touring consistently shows about 1% better MPG than I am getting. Since they calculate MPG based on fuel metering through the injectors, I'd expect manufacturing tolerances would be responsible for the errors.

 

<div class='quotetop'>QUOTE(Marlin @ Jul 11 2007, 10:18 AM) [snapback]476817[/snapback]</div>

Except that 99.9 is not the Prius's representation of infinity.

99.9 is the Prius's representation of any MPG number larger than 99.85.

So if you are using about a teaspoon of gas for each 722 feet traveled (about 105mpg), you will see 99.9.

Everything else in this post is my personal opinion and assumption. Perhaps someone with more knowledge can either confirm or correct my thoughts.

Note that the MFD doesn't update continuously. I'm not sure exactly what the interval is, but it seems to update about once per second or so.

I've assumed that if any gas was used at all during that second (a hundred or so compressions in the ICE, each one with an opportunity to give a little push), then the display would show orange arrows.

I've also noticed a delay (another second or two) between when I can feel something like the engine starting or stopping and when it shows up on the MFD. I've assumed that the "instantaneous" information on the MFD is actually more like the most recent running average of the past few seconds.

 

>Is it just a whisper of gas being used, such that it doesn't register enough to count as consumption?

>>99.9 is the Prius's representation of any MPG number larger than 99.85.

Danny, I didn't even consider that 1-decimal display not being able to do anything but round up or down! duh!
Makes for a very reasonable explanation, and could just as easily confirm my assumption that barely a hint of gas was being used in the ICE for that nearly-1-mile stretch of road, to maintain the speed, but not enough to trigger a lower-than-99.85 mpg calculation result, which would have displayed a 99.8 (or lower) value.

 

<div class='quotetop'>QUOTE(ChristoB @ Jul 11 2007, 01:50 PM) [snapback]477008[/snapback]</div>

I guess it comes down to how you define "a whisper of gas" or "a hint of gas".

At an actual 99.9 mpg you are using about 1.3 fl.oz. (2.6 TBSP or 7.7 tsp) for the mile.

You don't drop below 1 fl.oz per mile until you reach 128 mpg. (1 TBSP per mile at 256 mpg, 1 tsp per mile at 768 mpg)

 

<div class='quotetop'>QUOTE(Danny Hamilton @ Jul 11 2007, 01:05 PM) [snapback]477062[/snapback]</div>

For what it's worth, the Scan Gauge II that I have in my Prius does indeed display MPG to three digits, so when coasting down a hill with the engine idling I notice I get 200-300mpg at that time. When the engine is off and I am coasting down a hill, or in EV mode, it will display 9999mpg.

 

<div class='quotetop'>QUOTE(pkhoury @ Jun 18 2007, 05:11 AM) [snapback]463536[/snapback]</div>

Hi Paul,
Sorry for my late response. I missed your post for a while...

My SuperMID M-1 measures the fuel injector pulse width for fuel usage and counts speedometer pulse for distance traveled.
It has the RS232C output then it works together PCM software.
http://priuschat.com/index.php?showtopic=25759&hl=

<div class='quotetop'>QUOTE(ChristoB @ Jul 12 2007, 12:25 AM) [snapback]476823[/snapback]</div>

We observe 70 to 80 km/L on that situation which means 165 to 188 mpg.

<div class='quotetop'>QUOTE(wiiprii @ Jul 12 2007, 05:23 AM) [snapback]477072[/snapback]</div>

The ScanGauge II has some limitations for the fuel-cut mode on highway.
http://www.cleanmpg.com/forums/reviews/t-c...ge-ii-4495.html

"Another issue has to do with Fuel Cut. The ScanGauge II has no way to tell if the ICE is in Fuel Cut or if the car is at speed running at idle as you would in a NICE-On. Although the FE calculated and shown is very high under this condition (between 130 and 250 MPG depending on speed and ICE size), fuel is still shown being consumed when it is not thus causing an ever increasing error to build in. "

It is very hard to judge whether real or fake fuel usage by ScanGauge.

Ken@Japan

 

Just a side note on ScanGuage mpg: I've found that if I start in EV mode and go to ~20-25mph then go into a glide, the SG will register up to ~4,500 mpg then will go back to 0 and start counting up again.