Lifetime MPG

Discussion in 'Gen 2 Prius Fuel Economy' started by tmgrl3, Aug 3, 2006.

  1. tmgrl3

    tmgrl3 Member

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    Does anyone just leave the energy monitor running without reseting after each fill-up?

    With all of the discussions I have read so far and discrepancies between the monitor mpg readout and the actual mpg from fill-up to fill-up, it seems like a good idea to always reset...

    Besides I want to to just log the first tank (which is only giving me 31.8 mpg so far) and move on...

    I had the car running in IG-ON for a long time while setting up phones and phone book ....also didn't know some of what I am learning now about how to build up mpg.

    I actually got "dead screen" today for short bursts under 30 mph!! Such fun!

    Think they'll pass a law like they have about cell phones...

    No constant watching of Prius's energy monitor while driving...:D :D :D
     
  2. wstander

    wstander New Member

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    <div class='quotetop'>QUOTE(terri @ Aug 3 2006, 07:47 AM) [snapback]296940[/snapback]</div>
    If I recall, the MPG screen resets automatically after you put 6-7 gallons in; I don't think that you can override this and have it continually run.

    What I do is record what the screen monitor indicates, and then reset it manually. A few times I forgot, and true to form, it reset by itself.

    Anyway, keep a log of indications of MPG and note the mileage. You can then compute the gallons used by the engine and note a running total versus miles driven.

    Say you drove 300 miles and the MPG on the MFD says 50.0; you used 6 gallons. Next time, you go 360 miles and MFD shows 45.0; you used 8 gallons.

    Your lifetime is 660 miles divided by 14 gallons = 47.14 MPG

    gallons used miles driven lifetime mpg

    6 300 50
    8 360 47.14

    I just copied someone else's spreadsheet and modified it for my needs.

    As has been noted, the bladder makes the oldschool methods a bit unreliable due to not ever being certain haw much gas is being used; others noted that the MFD uses the amount of gas actually injected in to one cylinder and manipulates THAT into the numbers displayed.

    In over 28000 miles on my 2005, my lifetime is 49.4 MPG.
     
  3. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    YES!! I realized this yesterday evening.

    MFD displayed a tank average of 61.7 MPG.
    I drove 574.2 miles on that tank.
    I pumped 10.782 gallons of gas.

    Conventional wisdom would tell you that it took me 10.782 gallons to drive 574.2 miles since that's the amount of gasoline it took to replace what was gone. Conventional math woud tell you that I actually averaged 53.26 MPG (574.2/10.782). I don't need to remind anyone here that those are two very different tank results.

    Nonconventional wisdom and Prius experience holds that since it was approximately 95ºF when I pumped the gas, the temperature might have allowed the bladder to stretch during pumping. This would mean that I not only replaced the amount of gas I used, I added to it. The addition of more gasoline throws off my variables and therefore my calulations.

    When I'm indicating the tank averge, I go straight off the MFD because, as wstander said, the Prius knows better than I exactly how much gas was actually used to travel those miles. But when portraying my lifetime average, I still use the "Total-Miles" divided by the "Total-Gallons" technique because I believe the averaging over many tanks will level it all out.

    For lifetime results, click the link in my signature.
     
  4. wstander

    wstander New Member

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    <div class='quotetop'>QUOTE(TonyPSchaefer @ Aug 3 2006, 10:09 AM) [snapback]297045[/snapback]</div>
    Hm, I almost always fill up at 0515 and the ambient temp is between 45-65 degrees F. The car is always warm from my 47 mile one-way commute. How much stretching is going on?

    Also, I notice that the mileage driven is always +/- 1 mile difference from the odometer reading, ie;

    my MFD may state 307 miles, but the main odometer reading may indicate that I have driven 308 miles. Doing the math, the 2 are never the same.

    Ideas?
     
  5. San_Carlos_Jeff

    San_Carlos_Jeff Active Member

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    <div class='quotetop'>QUOTE(wstander @ Aug 3 2006, 10:09 AM) [snapback]296961[/snapback]</div>
    I've heard that the 2006's don't reset automatically. My 2005 does just what you say.
     
  6. wstander

    wstander New Member

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    <div class='quotetop'>QUOTE(San_Carlos_Jeff @ Aug 3 2006, 11:03 AM) [snapback]297081[/snapback]</div>
    Aha! If that is the case, then the OP is 'home free' and should just 'let it ride' to get lifetime.

    OTOH: I am sometimes fumble fingered enough to possibly hit the reset when I chnge screens, sooooo..... B)
     
  7. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    It would be almost as easy and fool-proof to just record the total number of gallons ever pumped into the car. With that information and the Main ODO, you can calculate Lifetime MPG in a couple seconds.
     
  8. berylrb

    berylrb Member

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    I'm like wstander and fill up usually when the temp is 50ish, my average fill up is 6.16 gallons and I have 132 tanks so far, I hope all the bladder issues have evened out over this many tanks!

    hey question for you guys, do you think there's any correlation between not waiting as long to fill up and this discrepancy in gallons due to the bladder expansion? I mean what if one filled up at half a tank for instance?
     
  9. john1701a

    john1701a Prius Guru

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    Don't forget there will always be pump issues. Unless you use the same one all the time, there will be a difference. There is no consistency of what level they detect "full" at.

    To even further complicate matters, the sensor in the pump only lasts about a year. At that point, it won't even be consistent with itself. They wear out after awhile.

    In other words, lifetime is the ideal. Measurements from tank to tank, even without a bladder, are not reliable. Use the Multi-Display for that. It's pretty darn close.
     
  10. barbaram

    barbaram Active Member

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    I don't know what to think- I've always been in Tony's club ... my display has been showing 48.2 more or less for some time now- I do not hit the reset.
    my spreadsheet of miles/ gallons shows 46.6 lifetime...... so does it really matter that gallon so so discrepancy?
    in the big picture... not really! stll far better that the old saturn!
     
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