1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Which matters more?

Discussion in 'Gen 2 Prius Fuel Economy' started by kdmorse, Jan 20, 2006.

  1. kdmorse

    kdmorse Member

    Joined:
    Oct 15, 2005
    205
    7
    0
    Location:
    Germantown, MD
    Vehicle:
    2006 Prius
    The question is purely academic, since I can't control either - but I was was wondering.. .

    Which has the greater effect on your MPG, how cold it get's overnight, or how cold it is when you start driving?

    Is it the cold overnight temperature, which saps every little bit of retained heat out of the engine compartment, that hurts during the winter? Which would make sense, as it would completely counteract anything the car does to attempt to save heat away at shutdown for the next startup.

    Or is it the actual running temperature when you get going that matters more, the actual cold air environmental temperature as you drive about.

    For example, this week it has been in the 60's during the day, and the 20's at night, with fairly sudden transitions. If I drove the car during the day, in the 60's, would my MPG be more summery, or wintery?

    These are the things I wonder about as I wait for my car to arrive....

    -Ken
     
    #1 kdmorse, Jan 20, 2006
  2. Jack 06

    Jack 06 New Member

    Joined:
    Oct 13, 2005
    2,556
    0
    0
    Location:
    Winters, CA: Prius capital of US. 30 miles W of S
    Based on my experience, it's the ambient temp when you drive it that counts.

    Your engine will warm up to the same operating temp whether it goes down to zero or 20 below---it'll just take a little longer from 20 below.

    You can best test this at a steady highway speed in different temps. Mine does better at 95 degrees than 80 degrees. At some temp it just won't do any better, but I don't know what that temp is.
     
    #2 Jack 06, Jan 20, 2006
  3. hdrygas

    hdrygas New Member

    Joined:
    May 22, 2004
    3,650
    6
    0
    Location:
    Olympia Wa
    Vehicle:
    2004 Prius
    What you want to know is the coolant and engine temp at start up. The block and coolant may be different from ambient temp. The warming cycle from the stored coolant mainly warms the header and the intake not the entire block.
     
    #3 hdrygas, Jan 22, 2006
  4. jeromep

    jeromep Member

    Joined:
    Dec 8, 2004
    827
    2
    0
    Location:
    Eastern Washington State
    Vehicle:
    2005 Prius
    Model:
    N/A
    I think it is fair to say that both conditions you mention are in effect when MPG drops during the winter. If it takes longer to come up to an optimal block temperature, than means you are burning fuel longer and with less regard to operational efficiency. But so does the ambient temperature. Air more dense at lower temperatures. Which means that there is greater air resistance for the car to slice through as it moves. So, even if the car is operating at its optimal block and cat temperature the vehicle is still having to fight its way through thicker air.
     
    #4 jeromep, Jan 22, 2006
  5. Frank Hudon

    Frank Hudon Senior Member

    Joined:
    Jan 11, 2004
    4,147
    18
    0
    anybody on the board who has pulled all the wheel bearings and cleaned them and replaced the grease with synthetic grease? Wheel bearing grease at -10 will have the consistency of wet cement just as it sets. Further increasing drag.
     
    #5 Frank Hudon, Jan 22, 2006
  6. efusco

    efusco Moderator Emeritus
    Staff Member

    Joined:
    Nov 26, 2003
    19,891
    1,192
    9
    Location:
    Nixa, MO
    Vehicle:
    2004 Prius
    Model:
    N/A
    First of all, you can control the overnight temperture by adding an Engine Block heater which does a fine job of heating your ICE to around 133 degrees or so which dramatically improves your mileage in the first 5-10 or even 15 minutes depending upon your commute.

    Next, I think the cold ICE has a greater impact for short commutes or commutes that start with multiple stops in the first 10-15 minutes--the ICE never has a chance to get up to normal operating temps so your mileage is terrible during that entire time.

    If your commute is long then it's probably the ambient driving temp that ultimately matters more....though adding an EBH wouldn't hurt there either.
     
    #6 efusco, Jan 22, 2006
(You must log in or sign up to post here.)