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Poor MPG upon returning to low Altitude

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Ed Carmack, Sep 25, 2013.

  1. Ed Carmack

    Ed Carmack New Member

    Joined:
    Aug 28, 2013
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    Vehicle:
    2005 Prius
    Model:
    Two
    Have an O5 with 165K miles. Just completed a 6500 mile trip. From the southeast to Oregon, down through California high country back via AZ, NM, and Texas. Around 3000 miles of high altitude driving. Car did fine with around 50 mpg average. On return as soon as I dropped down to low altitude in west TX mileage went from 50ish to 42. Was running mostly at 75. Very noticeable change. Mileage stayed that way until got home 1200 miles later. Slowing down to 65 did not help much at all. Monitor and gas pump in agreement. Mileage sucked. Otherwise car ran fine. Wind was not a factor. If anything mostly had a tailwind.

    I am guessing that the car computer had adjusted for high altitude and did not reset upon returning to low. Disconnected 12v battery negative terminal and let it reset to default. Short test drive. Let car warm for 5 miles and reset monitor. Drove 7 on interstate at 65mph and 4 on city roads. Monitor says 67 average. Seems to have worked.

    Anyone have any ideas on cause or similar experience?

    Thanks






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    #1 Ed Carmack, Sep 25, 2013
  2. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    17,436
    10,258
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    Location:
    Western Washington
    Vehicle:
    Other Hybrid
    Model:
    N/A
    Higher altitude means higher mpg, because the thinner air creates less air drag at highway speed. This was very noticeable in my older nonhybrids with greater drag.

    The thinner air also reduces 'pumping loss', one of the inefficiencies of Otto-cycle (traditional gasoline) engines. But this effect is much less pronounced in the Atkinson-cycle engine of the Prius.

    Speed is very definitely a factor, see Updated MPG vs MPH chart for the GenIII. Your GenII will be a bit lower, particularly at the higher speeds. Notice that even GenIII cannot get 50 mpg at 75 mph (steady speed, no wind or drafting, low elevation). If you didn't notice an improvement when dropping speed for one segment, it is mostly likely that something else interfered on that segment.
     
    #2 fuzzy1, Sep 25, 2013
    krmcg likes this.
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