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37 MPG highway (manually calculated)

Discussion in 'Gen 2 Prius Fuel Economy' started by Yoda-chan, Jun 6, 2008.

  1. Yoda-chan

    Yoda-chan New Member

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    I'm sure hoping someone can help us with our MPG dilemma.

    We have an '08 that's nearing 2000 miles. The car has been consistently getting between 57-59 city, and around 50 highway (no a/c). We took the car to Miami (a five-hour drive) two weeks ago, and the car got 46 MPG, with a/c and cruise control running.

    The MFD and the calculated mileage have always been within 1-2 miles of one another, including on the trip to Miami.

    Coming home, we filled the tank to the top (from half-full) at a rather sketchy Sunoco station in Dania. It seemed like the car was guzzling gas, although the MFD was reading 46 MPG again. However, the fuel indicator was dropping much faster. We filled the tank when we got home, and it took 8.3 gallons, yielding a MPG of 37 or 38.

    The trips to Miami and from Miami were similar. We used the a/c (set at 82) both ways, and speeds were similar. We drove 2 MPH (71 versus 69) faster coming home but didn't use cruise control. There was no wind or traffic in either direction.

    We're concerned that the car would (1) perform so poorly compared with its other trips and (2) have a noticeably different calculated mileage than that on the MFD. Our gut feeling is that it was bad gas, or perhaps an incredibly high ethanol content. Any thoughts would be appreciated.
     
    #1 Yoda-chan, Jun 6, 2008
  2. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    Currently, you have 45 posts under your belt. So I'm assuming that you've been around enough to know that you simply can not do manual calculations for Prius mileage. Regardless of the closeness of the other tanks. If anything, you have created your own example of why this is not a good idea.

    Then again, I'm not ruling out a high-ethanol content gasoline from the "sketchy" Sunoco station.
     
    #2 TonyPSchaefer, Jun 6, 2008
  3. Yoda-chan

    Yoda-chan New Member

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    Yeah...I'm still learning that you can't do manual calculations. It's ingrained into my head from my old Civic, and hard to get away from.

    It was just the expanse of the discrepancy that concerned me--and how fast the fuel tank seemed to be dropping on the highway.
     
    #3 Yoda-chan, Jun 6, 2008
  4. morpheusx

    morpheusx Professor Chaos

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    Your MFD is more accurate then your hand calculations. You would need to spread a hand calculation over several tanks to balance out the inaccuracies of full tank.
    It is quite possible that you put more fuel in the second time especially since you likely didn't fill from empty so the bladder was stretched a bit.
    8.3 gallons - 6.7 gallons = only a difference of 1.6 gallons

    Here is how
    37 X 8.3 gallons (Pump) = 307 miles (hand calculation)
    307 miles (ODO) / 46 MPG (MFD) = 6.7 gallons (MFD Calculations)
     
    #4 morpheusx, Jun 6, 2008
  5. Devil's Advocate

    Devil's Advocate Member

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    Yep, due to the inability to determine how much fuel is ACTUALLY in the Prius tank at any one time (including a fill-up) manual calculations have to be spread over a large number of re-fuelings.

    I currently have 401 re-fuelings with the following MPG's:
    Screen Display: 41.378
    Calculated MPG total miles / total Gallons of fuel: 40.611
    Calculated average MPG from each re-fuel: 40.568

    Most of my driving is Freeway (about 90%) from Vegas to SoCal, so over 3, 4000 ft peaks and averaging 80 MPH, and I sometimes tow my motorcycle and my girlfriends motorcycle on a trailer with the Prius.
     
    #5 Devil's Advocate, Jun 6, 2008
  6. BobA

    BobA New Member

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    a lot depends on how the tank is filled.. was it topped off this time or last time.. what time of day did you gas up.. I keep a book on my fillings.. sometime the calculator wins other times it's the MFD.. either way I'm still getting between 39 and 42 mpg.. all in town driving short drives and lots of stops..

    Bob
     
    #6 BobA, Jun 6, 2008
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