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    dianeinreno New Member

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    I find that an interesting correlating report - thanks :) . I'm tempted to go back the same way tomorrow and see if it happens again although I don't believe it will in any way convince the "mysterious headwind" believers :) . I've noticed several anomalies in the extreme heat. My shift lever went wonky which I put in another thread and several times I noticed that after 10+ minutes of driving with charge going to the battery all the while - the battery stayed at 1 bar. I stopped and tried to charge it by using brake and pedal but it still stayed at one charge until the temp went lower (it was 115 outside at the time). There are definite anomalies in the Prius at extremes of heat and I don't find it surprising that altitude (below sea level) could also contribute its own set of anomalies.
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    bisco cookie crumbler

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    i've never taken the pri to fl, but have been there many times in our hycam. mpg's never changed from virginia to fort myers and back. 40-45 all the way.
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    fuzzy1 Senior Member

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    If you are going to repeat it, I'd also suggest trying it in cooler conditions. The heat triggering some sort of battery protection would seem like a much more likely cause of low mpg than would low altitude.
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    bwilson4web 03 and 10 Prius

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    If you get a chance to repeat the test:
    • run 10 mile segments at different speeds - this will give a clue as to whether it is overhead, rolling, or aerodynamic
    • make sure you have a reference to compare
    Now in a perfect world, you would have an OBD recording system to measure ICE rpm, mass-air flow, MG1 torque and other metrics. We do not have enough information to tell if is engine, transaxle, tire, or other.

    Bob Wilson
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    dianeinreno New Member

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    I decided to go back the same way today. I'll compare to flat 50 mile segment that is 200 feet plus above sea level to the flat 60-80 mile segment that is 200 feet below sea level. I'll be going at a constant speed on cruise control at 65 mph

    I am not going to do different speeds because I do not see how that would matter and in fact just adds another variable to the mix. Also I am not going to do 10 mile segments because that really does not give enough of a baseline to really determine overall mileage. Just as before everything will be constant, temperature ( to within 5 degrees) , speed , type of gas in the tank.

    I'll be comparing a 30 mile stretch from Yuma, Az to El centro along Highway I-8 t(about 200 feet above sea level) o a roughly 40 mile stretch from Brawley to Thermal along I-86 )from 100 to 200 feet below sea level) . I'll reset the mpg counter during each stretch and we'll see if it repeats or not.
    bwilson4web likes this.
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    bwilson4web 03 and 10 Prius

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    Just a minor technical point about the "10 mile" segments. I've found over the years that the indicated MPG achieves a steady-state over the same route, direction, and temperature. Anything less and there can be too much variability. Longer is better but I don't have the time to waste.

    Also, the reason for different speeds, especially if we can get two extremes:
    • high-speed - predominated by aerodynamic drag, the rolling resistance remains constant
    • low-speed - predominated by rolling resistance with less aerodynamic drag
    We know the drag force is proportional to (from memory so I may get some wrong)
    1. drag ~= air_density * cross_section * coefficient_of_drag * (velocity ** 2)
    2. rolling drag ~= coefficient_of_friction * weight
    3. transmission loss ~= constant_per_transmission * viscosity * velocity
    In the past, many used just the first two terms. However, the EPA/DOE uses the three parameter formula. Regardless, the old-school, two term is good enough. Two data points are generally enough, if accurate, to derives the coefficients for the two parameter formula. But usually an instrumented roll-down test is used (which can be done using a recording GPS such as a Garmin) because it gives data points for the car drag without the engine. . . . now I'm techno-babbling.

    What ever data you can provide will be appreciated and give insights as to what is going on. The more data, the better, but we'll survive. <grins>

    Bob Wilson
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    fuzzy1 Senior Member

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    If you have a GPS or topo map available, please note the elevation of the start and end points of each measured segment, not just the average or typical elevation along each segment. Prius mpg is very sensitive to even small climbs or descents, so we will need to make some adjustments.
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    Andyprius # 1 Guest

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    Strange, nobody ever mentioned using the AC ?

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