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I'm impressed!

Discussion in 'Prius v Fuel Economy' started by Joe Fisher, Nov 19, 2015.

  1. Joe Fisher

    Joe Fisher Junior Member

    Joined:
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    Location:
    Kalispell, Montana
    Vehicle:
    2015 Prius v wagon
    Model:
    Two
    I just filled up tonight, I got 42.1 MPG (hand calculated. The onboard showed me at 44.9)

    The impressive part is the weather is pretty much the worst for good mpg. It's cold, below freezing, icy, I have studded tires. I teach in Marion, 15 miles and 1,000 feet up a mountain away.

    I was quite surprised at the difference speed made. My prior tank was 31.8 mpg.

    For both tanks the terrain was similar, the largest difference was the 31.8 tank was done at highway speeds of 70-75 mph.

    My 42.1 tank was mostly highway miles also, but at speeds of 55-60 mph.

    The car is new, it's a 2015 v with less than 1,000 miles on the odo.
     
    #1 Joe Fisher, Nov 19, 2015
  2. Air_Boss

    Air_Boss Senior Member

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    Vehicle:
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    Five
    Great teaching example. Aerodynamic drag is a speed-squared phenomenon, so faster is thirstier, with the power (and thus the fuel flow) required varying as speed-cubed. 75mph vs. 60mph being a 1.25x factor, the relative drag at 75mph is 1.56x that at 60mph, and the relative power/fuel flow is 1.95x that at 60mph.
     
    #2 Air_Boss, Nov 19, 2015
    wjtracy likes this.
  3. mikefocke

    mikefocke Prius v Three 2012, Avalon 2011

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    Tank shut offs vary. And the engine is getting broken in and you are getting used to it. But the important thing is you are pleased.

    air_boss, what is the difference in 5 mile increments? If I choose to go 70 in a 65 (am I right ~16% worse or maybe ~4 MPG?)?
     
    #3 mikefocke, Nov 19, 2015
  4. Air_Boss

    Air_Boss Senior Member

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    Five
    70/65=1.16

    1.16^2=1.35

    1.16^3=1.56

    [edited]

    Instantaneous power required (call it fuel flow) would be 1.56x but instant mpg would be 1.35x (1.56x fuel flow per hour but 1.16x faster, so mpg hit is 1/1.35=0.74 times prior mpg,

    If mpg at 65 was 44, it is now ~33; if it was 48, it is now ~36.

    IOW, a big instantaneous mpg hit on the speed-squared drag rise and speed-cubed power demand.
     
    #4 Air_Boss, Nov 20, 2015
    Last edited: Nov 20, 2015
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