Effect of non-ethanol fuel on indicated vs calculated mpg ?

Discussion in 'Gen 3 Prius Fuel Economy' started by Robert Holt, Aug 22, 2017.

  1. Robert Holt

    Robert Holt Senior Member

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    When refueling, I usually have a displayed mpg of 56-57 and a calculated mpg of 53-54, so typically a 3-4 mpg difference. With non-ethanol gasoline this difference seems to dramatically decrease. I first noticed this effect when driving in Alaska where non-ethanol gasoline is the norm two years ago. But earlier this Summer I refueled with non-ethanol fuel at a Stewart's Sweet Shop in upstate New York, and 550 miles later when I refueled the MFD displayed mpg was 60.1 and the calculated mpg was 59.9, a surprisingly small discrepancy. That brought back the memories of a similar shift using the non-ethanol fuel in Alaska, where I ultimately had to recalibrate the fuel consumption estimates of my Scangauge II in order to have the estimates better conform to actual consumption .
    Has anyone else noticed a decrease in the discrepancy of displayed vs calculated mpg in a Gen III Prius when using non-ethanol vs 10% ethanol fuel? Thanks in advance for relevant info!
     
  2. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    I did one near complete fillup with ethanol free (Chevron 94 octane), and the error on that tank was not significantly different, 6.5%. I refilled around half-empty with Chevron regular, so sort of a mix in there now, and this tank is displaying 3.9 right now, which is kinda phenomenal for us, not sure what's going on.

    IMG_7561.JPG

    3.9 liter per 100 kms likely bodes for a calculated tank around 4.2, which translates to 56 mpg (US). This is with 17" Michelin Pilots.
     
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  3. Mendel Leisk

    Mendel Leisk Lapsed Cargo Cultist

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    ^ At fill up displayed was still at 3.9, and calculated was 4.2, as I'd expected. Again, this was roughly a 50% ethanol-free tank.

    Fill up was again with regular, 87 octane chevron, with ethanol, and displayed has so far settled to around 4.5. I'd speculate more or less back to normal ethanol percentage.

    Not a lot of miles; I suspect if I persevere I might at best make 4.3 displayed, 4.6 calc'd.
     
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