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Prius at high altitude, interesting observation and a question

Discussion in 'Gen II Prius Fuel Economy' started by pt95148, Sep 8, 2010.

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  1. pt95148

    pt95148 New Member

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    I recently took a job in Reno and drove back and forth to San Jose for long weekends in my 08 Prius. I observed the followings.

    1. Whenever I climbed above 2k feet going to Reno, the fuel gauge dropped 2 bars? My thinking is that the air pressure drop caused the bladder to expand? Judging by the empty water bottle being crushed when I reach SJ, the opposite to climbing?
    2. The ICE seems peppier in town at 4.5k ft? Any reason or my imagination?

    My mpg dropped .20 mpg so far at 47.8, contrary to other reports of high alt driving in the forum. Going down to SJ makes no difference???
    True, I keep speed at around 75mph both ways.
    It was 48.9 mpg when I was commuting 12 miles each way with long coasting, then over the last year it has been dropping steadily to 48 mpg before going to Reno.

    Car was serviced regularly with oil and filter change, tires around 40 psi and with 22k on the clock.
    Can someone help me understand the steady drop as well as the 2 points above?

    Cheers
  2. fuzzy1

    fuzzy1 Senior Member

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    I can't comment on your GenII issues, but will on two other matters:

    (1) If your 'mpg dropped' is over the life of the car or other long distance, it is meaningless without an indication of what that distance is. This doesn't sound like a per-tank measurement;

    (2) My non-hybrids always produced significantly better mpg at high altitude, due mostly to less drag in the thinner air, and lower engine 'pumping' loss. The Prius still benefits from the former, but less so from the later due to efficiency measures that sharply reduce it at all elevations.
  3. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    In comparison to other cars, the Prius may feel peppier at altitude. The electic motors are un-effected by altitude while the ICE and almost all other cars are effected by reduced oxigen available. Turbocharged cars have a similar benefit, you can tune them to get the same amount of intake air at altitude they do at sea level.
    1 person likes this.
  4. pt95148

    pt95148 New Member

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    It has been dropping from 48.9 to 48 over regular commute 12 miles one way and then to 47.8 with the 260 miles trip to Reno. The mpg is the computer readout, not per tank measurement.
  5. dokeefenm

    dokeefenm New Member

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    This isn't specifically related to altitude, but do people know if the factory air filter is better for mpg than and after market?
  6. qbee42

    qbee42 My other car is a boat

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    The factory air filter works as well as any when it comes to mileage. The engine on the Prius is hardly ever worked hard enough to have restricted breathing.

    Tom
  7. dokeefenm

    dokeefenm New Member

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    Thanks, but is it best to use the factory air filter rather than after market? I've read some posts here that seem to say that a filter that allows too much airflow can affect the fuel air mixture and confuse the computer,resulting in poorer performance.
  8. jdenenberg

    jdenenberg EE Professor

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    Nonsense! Cars have a Mass Airflow Sensor (MAF) that measures the quantity of air flowing into the intake manifold so the computer can meter the correct amount of fuel for the required power.

    But as noted earlier, the factory air filter is fine in a Prius. Just change it out when it gets too dirty (usually at about 30,000 miles).

    The way to confuse the system is to have an air leak after the MAF and the computer is given the wrong data (A hole in the air duct did this to my 1991 ford probe a number of years ago - difficult diagnosis - easy repair).

    JeffD
  9. dokeefenm

    dokeefenm New Member

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    Thanks...I know about the airflow sensor,but thought there may be something different w/ this little engine, and then saw some posts discussing it.

    As to the other part of my question, I always buy after market filters and wonder if they are as good for the Prius or not,or if the factory filter would deliver better performance and cleaning?
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