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MPG Help

Discussion in 'Gen 2 Prius Fuel Economy' started by rolin, May 16, 2008.

  1. rolin

    rolin New Member

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    Hello to all

    I am new here and enjoy reading these so very informative posts. I have concerns about my MPG and I would appreciate any help you might offer.
    Here's the deal. I purchases, and love, my 2008 Prius with package #7 3 weeks ago. Since that time, I have gained insight into the "Bladder" type of fuel tank, etc, and understand how to fill the tank, which by the way is very different than filling my Pathfinder, which I do Not miss...

    When I bought the car, the gas indicator had all bars lit, indicating a full tank. At 5 bars, I filled it up, with 5.013 gallons and set the trip A odometer to track my MPG. At 3 bars, the next week, I was able to fill it with 6.06 gallons and noted the Trip A odometer at 323.3 miles. I thought that I was supposed to divide the miles driven (323.3) into the 6.06 gallons. I was under the impression that I was getting approximately 53 MPG.

    What I am seeing now on my trip odometer is 168.7 miles driven, with now bars indicating 1/2 of a tank (5 bars). My consumption meter is showing today an average of 47.2 MPG at the 47 miles that I drove today.

    I am confused and I hope I don't sound a a bit ignorant here. How do I track my MPG, based upon the above MPG information, what does it seem to you that I am getting MPG? Can I be getting only 30 mpg? I don't think so, but the wisdom and knowledge I have see here is impressive.

    Many thanks

    Linda:)
     
    #1 rolin, May 16, 2008
  2. dogfriend

    dogfriend Human - Animal Hybrid

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    The problem is that you are assuming that the fuel gauge is linear (each pip is equal to a certain amount of gas). It isn't. That is why many here will refer to it as the guess gauge.

    Another issue is that the bladder in the tank makes it rather difficult to fill to the same level each time. So, you never know if you got it completely full or not. However, resist the temptation to fill to the brim - it is possible to damage the fuel system by overfilling - it is better just to fill slowly and stop after the pump clicks off the first time. Also, don't assume that you must have X remaining miles on the last pip because you got X mpg on the previous tank. Lots of people run out of gas by doing this. Better to be a little conservative and avoid running out.

    So, how to track mpg? The MFD is fairly accurate although most report that it reads slightly optimistic, maybe 1 - 2% higher than reality. You can still calculate the mpg by using miles driven / gal used, but you need to average 3 to 5 tanks to remove the error from not getting the same amount pumped into the bladder each time.
     
    #2 dogfriend, May 16, 2008
  3. pjberry

    pjberry New Member

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    I've only had to fill one time so far. But the actual fuel consumption rate (distance driven divided by amount of fuel added) gave me slightly better fuel efficiency than the display in the Prius, which is counter to what I've read here.

    Regardless, I plan on recording and resetting the value on the display with each fill, as well as record the calculated rate with each fill.
     
    #3 pjberry, May 16, 2008
  4. a priori

    a priori Canonus Curiosus

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    Please keep this information for the lifetime of your car and report back on occasion. You also can go to GreenHybrid.com (or other similar sites) and enter your data tank-by-tank. This will allow everyone to see your car's real-life mileage performance.

    You'll see variation between the MFD reading and your calculated numbers at every fill. Over time the variance will decrease, but there always will be some difference. I am at nearly 11,000 miles, and I see a difference of just about 1.4 mpg (the MFD reading is higher).

    One reason most people start out with a lower calculated mpg than MFD mpg is that they do not actually receive their cars with really FULL tanks of gas!
     
    #4 a priori, May 16, 2008
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