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calculated vs. computer MPG - Please post your results

Discussion in 'Gen III 2010+ Prius Fuel Economy' started by F8L, Jul 10, 2009.

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  1. F8L

    F8L Protecting Habitat & AG Lands

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    I am insterested in seeing how many people are experiencing wide swings in hand calculated MPG vs. computer derived MPG (FCD).

    I see a lot of high numbers from many new owners but it seems the ones who are tracking MPG with both methods are seeing wide swings of 6-10MPG less than the computer MPG. I would like to see if this is a common issue and the car is overestimating the MPG or if maybe there is an issue specific to say Dave's car or his fill-up stations.

    We have established through robust recordkeeping that the GenII is within 1%-3% or so from the MFD but I would like to see the same kind of data for the GenIII but since I cannot do it myself I am asking for you GenIII folks to perform the task. At least over a few tanks. :)

    Please post your MPG offset* (FCD MPG vs. hand calculated MPG) AND the month your Prius was manufactured**.

    * Offset is the difference between your FCD MPG and your hand calculations (divide miles driven by # of gallons pumped) Please ensure you have done this over as many tanks as possible. A few of tanks is not enough data.

    ** Manufacture date can be found inside the drivers side door jamb
    RetiredNavyMR likes this.
  2. a priori

    a priori Canonus Curiosus

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    I had thought I would wait until I had more tank fills to talk about this, but I'll try to put together a decent analysis of it and post it here.

    The short of it is that I am seeing some differences not unlike what I saw in my Gen2. My first fill up was dead on with the MID display, but I think that the later fills only showed that what I got from the dealer was something less than a full tank of gas.
  3. F8L

    F8L Protecting Habitat & AG Lands

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    That would be great. I look forward to seeing your results. With people like you reporting we don't have to worry about the accuracy of the information. Thanks! :)
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  4. alfon

    alfon Active Member

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    2010 Toyota Prius III with Navi

    With close to 5,000 miles I have noticed the MPG guage is off usually 2-3 MPG in the positive.

    My last tank was calculated by miles driven and gallons used at 54.6 MPG the guage read exactly 57.0 MPG.

    Alfon
  5. fuzzy1

    fuzzy1 Senior Member

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    I probably shouldn't chime in just yet, with only N=1 (57.1 displayed / 55.4 computed). The next month should push my N up to at least 5.
  6. cantgoback

    cantgoback New Member

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    This may be an old question but for newbies like myself, does anyone know why the MID MPG is higher than what someone computes from the miles and gallons? Is there some rounding error in the calculations? Or is there an error in the gas level gauge? Or something else??
  7. HTMLSpinnr

    HTMLSpinnr Moderator Staff Member

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    F8L - here's what my tracking charts look like thus far - the gap gets wider as display MPG gets higher.

    [​IMG]

    Attached Files:

    2 people like this.
  8. FireEngineer

    FireEngineer Active Member

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    Exact same thing I'm seeing Rick. As the MPG heads north of 60 MPG the gap gets wider between actual and HSI MPG. Somethings not right here, my Gen II did not have a wide discrepency on my mid 70's tanks.

    Wayne
  9. F8L

    F8L Protecting Habitat & AG Lands

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    Thank you guys for posting up. I think if we gather enough data that shows there may be a large discrepancy between the MID and the calculated MPG then we can persuade Toyota to pay more attention to this issue on future models or maybe make changes to the software currently handling the calculations. That is if there is indeed a problem.
  10. eldiee

    eldiee Member

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    On our 2010 V we show 54.0 for 1010 miles. Fuel used 19.5 gal. or about 52mpg. I guess it is a little bit high on the car read out. So far we have been really happy with the mpg.
  11. wave_slider

    wave_slider New Member

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    My friend is on his 3rd tank and his MID says 1.5 mpg higher than his calculation. I have 140+ miles and haven't even used a 1/4 tank yet, but I will be posting my results. I expect to use 2 tankfuls a month.
  12. sleewa

    sleewa New Member

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    Only 1 tank so far, but displayed MPG = 49.7; calculated MPG = 50.07

    2nd tank: displayed MPG = 51.0; calculated MPG = 48.00
  13. wvgasguy

    wvgasguy New Member

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    I also perform one other calculation when I do my manual averaging. I record the computer reading. From that I calculate the gallons the computer estimated I used based on the computer average and the actual miles to get a "theoritical" gallons used. From that and tracking the milage I am able to do a "running total" I.E. a lifetime total of the computer reading. That takes the variances out of the tank to tank reading. In the TCH I also made an adjustment for the odometer error. What I fouind was that my manual calculation with odometer corrections over the 67,000 miles of ownership was about 0.5 mpg below the computer lifetime average, but about 0.5 mpg above the manual average.

    Sounds complicated, but once the spreadsheet was set up all I had to do was enter the odometer reading, the cost of the gas (to keep a running operating cost as well) and the gallons used.
  14. wvgasguy

    wvgasguy New Member

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    Only one tank so far on the Prius, but I also found that when "hypermiling" the TCH I found that the variance between the manual calculation and the computer was wider. For example when I averaged 37 to 38 the difference was usually around 1 mpg, but when I averaged 40+ the difference was closer to 2 to 3 mpg. I attributed that to the error in measurement of the gas. I don't know what type of measurement is used but in most flow meters they do not have an accurate reading over the entire range of no flow to max flow. (I would assume they are not measuring gasoline used as you do in a gasoline pump but rather some sort of flow calculation). Thus high error readings on the computer are during times when there is a lot of EV function going on, or really low gasoline flows to the ICE.

    I never really had anyone smarter them me (that's a lot of people) confirm this possibility.
  15. ken1784

    ken1784 SuperMID designer

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    Japanese G3 owners are also seeing 5-6% error.
    I saw approx 10% error on the VW Golf. :mad:
    Are there any vehicle which reports the exact same computer MPG as calculated MPG?

    There are a lot of factors to produce computer MPG error, and a car manufacturers doesn't want to display worse computer MPG than calculated(actual) MPG.

    A rumor says that Toyota engineers are going to display intentionally 5% better number on computer MPG for the Gen3 Prius. :mad:

    Ken@Japan
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  16. Sphyrna

    Sphyrna Priusite

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    Just to add to the complicated hot mess that is calculating all this, I'm interested in what kind of driving and speeds. For instance I drive 75 Miles each way to/from work. about 17 are 'city' the rest are highway. The city streets average speed limit is about 40 and the highway spd limit is 65. Of course actual speeds vary day to day or even every few seconds! But there may be a correlation between the more accurate MID readings vs calc'd for slower or city driving.

    Figuring this out would give us a clearer picture of what this car realy does in different circumstances/styles of driving.

    I'm going to receive my Prius as soon as Mr. Obama buys my old clunker. It's waiting for me at the dealer, all the papers are signed and financing ready. [FONT=&quot]♫[/FONT] The waiting is the hardest part... [FONT=&quot]♪[/FONT]
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  17. sdembry

    sdembry New Member

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    I, too, am interested in seeing if the displayed error is consistently higher than manual calculations. I have measured manually twice (both after 350+ mile trips of the odometer, standard commutes over 1.5 week intervals) and have measured my actual (Odometer reading / full tank to full tank gals at pump) mpgs to be near 5mpg less than displayed. I called the service department to see if a calibration adjustment was possible. They said no adjustment, that they knew of.
  18. krisirk

    krisirk Junior Member

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    I'm getting about 3 MPG difference. Here's my first two tanks.

    Tank 1: 50.3 MPG Actual vs 53.7 MPG Computer

    Tank 2: 57.3 MPG Actual vs 60.4 MPG Computer

    Kris
  19. wave_slider

    wave_slider New Member

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    I filled up today at about 390 miles driven. My display read 52.4 and I calculated 49.9. It may also have been due to the fact that when I gassed the first time, I let it click off and then squeezed the handle halfway till it shut off again.

    I don't want to overfill so I will be stopping at the first click from now on. I will also fill at the same station, hopefully at the same pump to get a consistent pump shut-off.
  20. a priori

    a priori Canonus Curiosus

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    I had wanted to do this analysis after 10 tanks, but it is taking me too long to get there. :D

    I have filled the car 9 times, but I haven't used the first fill in my calculations, because I really don't know how full it was nor at what point the car was "filled" with gas before purchase. The 8 tanks I've purchased each resulted in calculated MPGs that were lower than the fuel economy shown on the MID readout. The variance hasn't been consistent, but I attribute that to my own inconsistency in filling the tank. I suppose I also can place "blame" on the gasoline pumps, also.

    The largest difference was 3.5 MPGs (57.6 vs. 54.1) while the smallest was 1.7 MPGs (50.7 vs. 49.0). The average difference is 2.6 MPGs (56.3 vs. 53.7) which is equivalent to about 4.8%.

    The following chart shows the tank-by-tank variance oscillates:

    [​IMG]

    Attached Files:

    Daddyjeph, F8L and IronWaffle like this.
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