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MPG style calculation

Discussion in 'Gen 2 Prius Fuel Economy' started by NuShrike, Mar 3, 2005.

  1. NuShrike

    NuShrike Active Member

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    I calculate my MPG by amount of gallons LAST filled / CURRENT trip miles.

    So if I filled up 1 gallon last, and I got 50 miles by the beep, then I got 50MPG.

    I've noticed other people (like john1701a.com) are calculating by amount of gallons CURRENTLY just filled up / CURRENT trip miles, but isn't this wrong and off by "one"?

    For example, you just got your Prius at 0 miles, and you filled the tank to 11.9gal, you certainly cannot calculate the MPG right then and there. You have to wait to the next fillup to see what the 11.9gal gave you, right? And then the math proceeds from there.

    Of course this style would require you to fill up ~ the same way every time. Hit the E/beep, run it to ~ same miles past the beep, and then fill up, or otherwise you'll get "better" MPG all of a sudden because you're digging into the extra "reserve" that was sitting there before your last fillup.

    Code:
    
real number example:

        miles  gallons filled  MID calc  (CURRENT) calc  (LAST) calc

current  344       8.300       45.2mpg      41.3mpg        43.17mpg

last     396       7.967       48.0mpg      49.7mpg          n/a

     
    #1 NuShrike, Mar 3, 2005
  2. Ray Moore

    Ray Moore Active Member

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    I'm going to have to fall next to John on this one. The method that you suggest will tend to be very inaccurate due to wild inconsistencies in the fuel guage. There are also inaccuracies in the level of individual fillups and in the pump meters themselves, but these hopefully level themselves out over time. It is more accurate to divide the miles you just drove by the gallons that you are pumping right now and try to fill to the same level each time.

    I trust the computer for tank by tank mileages and over 25,000 miles it has proven to be within 1% of correct as compared against my accumulated lifetime average that I compute by collecting all my gas receipts since the purchase of my Prius.

    As an example: tonight I filled up on the way home. The computer showed 642 miles at 57.6 MPG. The pump showed 11.2 gallons which calculates out to 57.34. The difference is statistically quite small. Fuel was visible in the neck at this fillup and the last. Both fillups were at the same pump and it was last inspected in January of 2002. I hope this helps.

    I guess I should get a life.
     
    #2 Ray Moore, Mar 3, 2005
  3. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    You're right in that the recording of MPG provides a historic view.

    When I track my tanks, I take the number of miles achieved on that tank and divide that number by the number of gallons required to fill the tank. This tells me it took 8.678 gallons of gas to drive 387.6 miles. Assuming that the newly filled tank is at the same level as the last time, I averaged 44.66 MPG. If you click on my signature, you can access my spreadsheet. If you look at my 2004 numbers, you can see that it took a tank before I could calculate mileage.

    I've read your post a few times and can't tell if we agree or disagree.
     
    #3 TonyPSchaefer, Mar 3, 2005
  4. vincent1449p

    vincent1449p Active Member

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    If you take the beep as reference, then everytime you must wait for the fuel level to drop below the beep for the same exact number of miles before you can refuel. If you refuel sooner or later, then your calculations will be off. OK, let's say you stopped exactly at the same exact number of miles and drive no further, but can you then always find the gas station where you stopped? Of course not, unless a gas truck is following you.

    On the other hand, if you use the top of tank as reference, you can refuel as and when you like. Yes, there may be some inconsistencies due to temp., bladder effect, etc but the errors will be even out after several tanks.

    Vincent
     
    #4 vincent1449p, Mar 3, 2005
  5. NuShrike

    NuShrike Active Member

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    Ya, you'll always have to fill from the bottom of the tank, but it shouldn't be off if you fill early, only late.

    Such as if I'm really low, I fill up 1 gallon of gas to make it to the next "favorite" gas station.

    LAST tank calculation gives me last fillup to current trip mpg correctly. CURRENT tank calculation would give an entirely inaccurate mpg.

    Then, I make the favorite gas station and fill up the full tank. Again, LAST tank calc would give me the right numbers where CURRENT tank wouldn't.

    So... whatever works. :)
     
    #5 NuShrike, Mar 3, 2005
  6. tim_in_jax

    tim_in_jax New Member

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    I know that this topic is pretty much dead ( no post in 2 weeks ) but I wanted to add my two cents in. My calculations of MPG are very similar to what Ray and Tony have described. The only difference is that I have added two estimated fuel columns to my spread sheet. In doing this I have made the assumption that a full tank is when the pump stops ( clicks ) that is considered to be 11.9 Gal. I know that I can get more gas in the tank but that would be an overfill ( that name alone suggests that that would make it over the 11.9 ). And if the pump click and I have only added 9 Gals that means that I consumed about 9 gals. I know that this method is subject to alot of inacurracies but I need some system to account for quick fill ups.

    Adding these two columns have helped my quick "1 gallon" fuel stops because I know what the "calculated" fuel level was.

    Again just my 2 cents worth.
     
    #6 tim_in_jax, Mar 17, 2005
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