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MPG on display? MPH on GPS?

Discussion in 'Gen 2 Prius Fuel Economy' started by Baldbubba, Apr 10, 2014.

  1. Baldbubba

    Baldbubba Member

    Joined:
    Feb 18, 2013
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    Location:
    in PRIUS down by the river
    Vehicle:
    2008 Prius
    Model:
    II
    how accurate is the display for MPG. I find it very difficult to gauge MPG with using the math via the pump with variance of the bladder capacity in the changing weather conditions. My gps and radar units setup along side the road show my speedometer on the Prius about 2 miles an hour fast. Will this make a difference in the MPG. The speedometer checks out on distance with the mile markers along side the road. I just want to know how all this plays into MPG if the speed is off but distance is spot on. I am able to get and HONEST (via the display) 42.8 mpg on tank. Thanks in advance for any knowledge you are willing to share.
     
    #1 Baldbubba, Apr 10, 2014
  2. Easy Rider

    Easy Rider Active Member

    Joined:
    Feb 9, 2014
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    Location:
    N. Central Florida
    Vehicle:
    2013 Prius c
    Model:
    Three
    The ONLY way to get true MPG fuel usage is to take amount used divided by miles travelled.

    The Speedo and Odo are different mechanisms or circuits. It is not unusual for one to be off but not the other.

    The MPG figure on the display is only an "estimate".

    Once you figure out what YOUR fudge factor is (most say 2 to 5 MPG high), it should stay pretty consistent and the relative number should be pretty accurate. That is, if the display says you are getting less mileage that "normal" then you probably ARE.......but you still can't trust the absolute number.

    And sadly, if you are having bladder problems, you can't even be sure how much you used since the last fillup.
    Just drive and don't worry about it.
     
    #2 Easy Rider, Apr 10, 2014
  3. dorunron

    dorunron Senior Member

    Joined:
    Jul 17, 2011
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    Location:
    Houston, TX
    Vehicle:
    2013 Prius
    Model:
    Three
    You will find over time that the bladder in the Gen II is going to make the MFD readings vary considerably and not always agree with the numbers at the pump. For accuracy, ignore the MFD and go by the numbers on the pump.

    Yes, the speedometer on the Prius is not perfect, but none are on any of the vehicles. When the speedo is working normally, the Prius will tend to read about 1 to 2 MPH faster than your typical GPS will. Again, use the numbers from the pump and the odometer on your car for accurate gas mileage calculation. (AKA MPG). The odometer is accurate IF you have the correct size tires and wheels on your Prius as per the tire placard that is on the car. You can verify the odometer by using specific odometer test posts on the highway. Exit marker numbers are not perfect miles, only the actual test posts that are seen occasionally could be used for odometer verification.

    Easiest way to track MPG - Login to Fuelly

    Best of luck to you,
    Ron (dorunron)
     
    #3 dorunron, Apr 10, 2014
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