Sorry I doubted you on affect of heat on MPG

Discussion in 'Gen 3 Prius Fuel Economy' started by DumbMike, Sep 26, 2010.

  1. DumbMike

    DumbMike Active Member

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    I read on many threads on these boards that heat helps our MPG. Grill blocking? Are you nuts? My brain could not compute all of this.

    Well, for the past few weeks the weather in Los Angeles had been a little on the cool side. I didn't notice the cooler weather, but I noticed that the computer says I was getting around 51 MPG for almost the entire tank (the warning was blinking so I think I used about 10 gallons).

    Then today, the temperature rose to around 90 degrees. I noticed the heat. All of a sudden, my computer MPG rose to slightly above 52. I think that would be difficult to do as it must average out the entire tank an extra 1 MPG. And correct me if I'm wrong, but if I got 51 MPG over 10 gallons of gas, in order to raise the gas milage by 1 MPG in the next gallon, I must get around 62 MPG for that last gallon. Correct?

    I fully believe that the heat is the only difference which caused the extra MPG in my case. So, I apologize again to everybody who said the heat would give me higher MPG, but I didn't believe you.

    I still don't like the idea of grill blocking, so I won't go that far. I'll just take the benefit of the hot California weather for the next few days and next summer.

    Dumb Mike
     
  2. vday

    vday Member

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    I am not sure how many digits after the decimal point the US model has but remember that 51.5 would show 52 and 51.4 would show 51 if there is no digit after the decimal point.
    Nothing you can do against the rounding effect.
     
  3. DumbMike

    DumbMike Active Member

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    Mine shows tenths. So I won't have any problem on rounding off to whole numbers. And I'll assume it is "semi" accurate to the tenths, at least to the extent to the relative accuracy of the computer, which for me is about 2 MPG higher than the calculated MPG.

    Dumb Mike
     
  4. SageBrush

    SageBrush Senior Member

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    Yep, 62 mpg for the last gallon.

    I waited until I had a scangauge before blocking my upper grill (G2 Prius) out of fear of overheating. I didn't even get close, in fact the radiator did not come on the entire winter. That said, if I lived in coastal Cali where 'cold' is 50F, I would not block either.
     
  5. Rokeby

    Rokeby Member

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    It's that time of year again. Air temps start to creep down, and we'll be
    seeing a quite few first-time summertime buyers seeing slowly
    dropping MPGs and wondering if their car is broken. It happens every
    year.

    There are members with Gen IIs who block one or both grills all the time.
    I'm one of the ones who has ScanGauge and keeps the upper grill
    blocked the year 'round.

    We've seen atypical late summer daytime temps in the high 80s-low
    90s, so I've held off putting the lower grill blocks in. I put the four foam
    pipe insolation strips in one at a time: At 70 degF daytime temps the
    first one goes in at the top. At each 10 degF drop, another goes in.

    I've also added some insulation to the engine itself, which seems to be
    a help combating heat loss as well. I'll put it in when temps hit the 50's:

    http://priuschat.com/forums/gen-ii-...radiant-heat-reflector-gen-ii.html#post979632
     
  6. john1701a

    john1701a Prius Guru

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    Cool?

    Try temps well below freezing, for the daytime high.

    Sitting in heavy traffic caused by snow & ice provides a very different perspective.
    .
     
  7. priustexasbob

    priustexasbob Member

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    ok, newb question time. why would the Prius get better mileage in hot/warm weather? I mean we have to run the a/c when it is hot which has to affect the mpg. in cooler weather, in se texas means <80, we won't run the a/c which should increase mpg. our weather is wacked here, we could have 50 for a high one day, 85 the next then 60 the day after that.

    my commute to work is about 4-5 miles at a max speed of 45 mph. I would ride my bike but with no shoulder and drivers who don't respect people on bikes it is too dangerous.
     
  8. alfon

    alfon Senior Member

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    Up here in the northwest winter is mostly rain 8 days a week. MPG always drop down into the mid 40's.

    al
     
  9. Rokeby

    Rokeby Member

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    The terms cold, cool, warm, and hot have meaning relative only to a
    posters particular experience and location. Given the wide geographical
    distribution of PRIUSchat members, they can be confusing, even meaningless.

    The Prius delivers the best mileage in the same temps we humans like.
    In general, the best temps for high mileage are between 50 and 80 degF.
    In this range most folks don't need to ask for either heat or cooling. As
    a result, with careful, attentive driving most, make that virtually all of
    the little gas that is burned goes to moving the car.

    High mileage follows. :rockon:
     
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  10. Joe166

    Joe166 New Member

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    Never mind.
     
  11. enerjazz

    enerjazz Energy+Jazz=EnerJazz

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    Here is my years of meticulous data. The car gets the best mileage when the temperature is warm, but not really hot.
    [​IMG]
     
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  12. macman408

    macman408 Electron Guidance Counselor

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    Nice data! Out of curiosity, what are your A/C habits like?
     
  13. DumbMike

    DumbMike Active Member

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    50? I start complaining when it gets to 65!

    I don't think I will ever block because I'm too old school and the idea of trying to keep an engine warm is just counter-intuitive for me. Maybe I'll overcome my old ways.

    Dumb Mike
     
  14. DumbMike

    DumbMike Active Member

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    I wasn't trying to rub it in. It was just our temperature in Los Angeles did cool off a bit.

    But you in MN get it real bad in the winter. Sorry.

    I'll keep checking my MPG, and definitely will be keeping my eye on it once the temperatures get "cold" for us (again, I'm not complaining). But I think my old age allows for some complaining.

    Dumb Mike
     
  15. DumbMike

    DumbMike Active Member

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    enerjazz,

    I, too, am curious how much you used your A/C when the outside temperatures get above 85. I didn't use my A/C much when it got hot yesterday, and I got real good gas milage that day. I drive to Las Vegas a few times a year when it's hot over there, so I was hoping to get some great MPG on my trips there.

    Dumb Mike
     
  16. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    EnerJazz's chart is a great temp-to-mileage comparison. I usually whip out my own chart to show the correlation. Originally, I didn't track temperature. A couple years into ownership, I noticed the up/down mileage pattern and decided to put temp in the background for comparison. Like you, Mike, that's all it took for me to become a firm believer.

    My cabin temp control habits are thus: A/C cooling when extremely hot (> 95ºF) and heat usually only when below about 25ºF. Usually at the beginning of the Winter, I use the heat and think "screw it, I want to be warm" but then I see my mileage drop and change my tune. One of these years I might decide that warmth wins over mileage (but don't hold your breath).

    [​IMG]
     
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  17. Downrange

    Downrange Member

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    I envision probably not driving the Prius as much in the coldest months. Since buying it in April, it's been the "go-to" car, but that may change.
    I'm also concerned about getting caught out in a snow event - something that happened to me in my RWD Volvo last year, due to weather forecast being off, and I don't wish to repeat that adventure in a non-AWD vehicle...
     
  18. enerjazz

    enerjazz Energy+Jazz=EnerJazz

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    On the question of my A/C habits . . .
    I live in the Dallas area and it can get pretty hot and sticky. In the summer I'll sometimes drive with the windows down in the morning, but my commute is mostly highway so when I get over about 45mph I usually roll up and switch to A/C to cut down road noise and drag.

    I do park in a parking garage during the workday, so my car only needs modest cooling when I commute home. If I had to park in the sun and cool it down it would be worse.

    When you see average temperatures in the upper 80's on my chart that means we are near 80 at night and about 100 in the day.
     
  19. DumbMike

    DumbMike Active Member

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    Just for clarification, it looks like your are getting your max MPG when it's around 100 degrees outside. Right? Then when it gets above that (say 90 degrees on your chart), maybe you are using your A/C a bit more, thus, your MPG lowers?

    And for those of you who wish a little misery on me, the temperature rose to around 113 degrees in downtown Los Angeles, about 111 in Santa Monica (by the ocean). I think those are record temperatures for any time of the year. I'll have to check. But I'm a little soft: I'll normally use my A/C when the outside temperature is 75 degrees, but I just didn't use it over the weekend for test purposes.

    Dumb Mike
     
  20. Colonel Ronson

    Colonel Ronson New Member

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    i think most of the misconception about temperature and fuel mileage comes from the fuel you get at different times of year.

    people tell me heat helps mpg. more like there is no ethanol in your gasoline during the summer, so you'll get more bang per gallon. people also say cold helps mpg, more like you don't run the a/c during the winter, helping reduce fuel usage.

    temperature and mpg is a bunch of different factors. i doubt its related. the internal temperature of the engine is the same regardless of outside temperature. touch your engine during the summer, and the winter. you'll burn it both times...lol.
     
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