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

MPG Display

Discussion in 'Gen 2 Prius Main Forum' started by Marvinh, Dec 25, 2009.

  1. Marvinh

    Marvinh Marvin

    Joined:
    Jul 2, 2007
    189
    7
    0
    Location:
    NY - Long Island
    Vehicle:
    2007 Prius
    Hello - I have been noticing a drop in MPG on cold startup (20-30 degrees fahrenheit). That is while the engine is warming up the MPG will drop about .2 tenths.

    After warmup the MPG will stabilize and come back up. I understand that the ICE will use more fuel while warming up, but I have never noticed an immediate drop in MPG, even while standing in my driveway!

    Has anybody else noticed their MPG go down on cold startup?

    Thanks! Marvin
     
  2. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    17,388
    10,238
    90
    Location:
    Western Washington
    Vehicle:
    Other Hybrid
    Model:
    N/A
    My MPG display drops far more than that, even in warm weather. If I reset it to get a measure of the next trip, it read "-.-" (meaning 0.0) until the car starts moving. And the initial numbers are pathetically small until the car gets warm and it gets several miles down the road.

    My guess is that you are not resetting the trip meter each trip, so it has a large number of miles already included on 'this' trip. Therefore it drops only slightly during warmup. Because my gauge usually includes zero to two previous outings, the warmup impact is far more noticeable.

    The display should drop during warmup. Just do the math. When you burn fuel without moving, G is increasing while M does not, so MPG must fall. And the smaller M and G are before you started, the greater the warmup impact.

    My non-hybrid equipped with a ScanGauge does the same thing. Except that its numbers are far worse, and it takes longer to warm up.
     
  3. Marvinh

    Marvinh Marvin

    Joined:
    Jul 2, 2007
    189
    7
    0
    Location:
    NY - Long Island
    Vehicle:
    2007 Prius
    fuzzy1 - thanks for your reply.

    I understand. And you are correct. I have been re-setting the trip meter at every fillup and lately I have been watching the display very closely because I had high performance tires installed (Michelin Primacy MXV4's). These tires give less MPG in stop and go traffic.. and much better highway miles. And of course winter temps and reformulated winter fuel makes a difference.

    I'm averaging about 38 MPG in the winter.

    So what I am going to do is NOT reset the trip meter at every fillup to see what my average MPG is over many tankfuls (combining highway and local travel). I expect the average to be above 40 MPG with all factors considered.

    Thanks! Marvin
     
  4. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    17,388
    10,238
    90
    Location:
    Western Washington
    Vehicle:
    Other Hybrid
    Model:
    N/A
    Does your 2007 have two trip meters, or only one?

    The 2010 has TripA and TripB, with MPG display for both. I use one as the tank average, and reset the other almost daily to get a shorter term measure. The later is necessary to get quick feedback about what works and what doesn't.

    And I keep a fuel logbook, to figure MPG over the longer term. Actually have been keeping fuel logs since 1984.
     
  5. Marvinh

    Marvinh Marvin

    Joined:
    Jul 2, 2007
    189
    7
    0
    Location:
    NY - Long Island
    Vehicle:
    2007 Prius
    My 2007 has two trip meters, but I was confusing the trip meters on the odometer with the MPG display on the Multi Function Display screen (MFD). Sorry 'bout that.

    So I am only talking about the MPG readout on the Multi Function Display.

    Marvin
     
  6. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    17,388
    10,238
    90
    Location:
    Western Washington
    Vehicle:
    Other Hybrid
    Model:
    N/A
    If your 2007 computes MPG linked to only one of the trip meters, then you won't have as much tracking flexibility as the 2010.

    Unless you install a ScanGauge. But that expense is much harder to justify in a Prius than in a non-hybrid that lacks built-in MPG display.
     
  7. richard schumacher

    richard schumacher shortbus driver

    Joined:
    Mar 27, 2004
    7,663
    1,040
    0
    Location:
    United States
    Vehicle:
    2004 Prius
    Model:
    N/A
    You're only noticing it now because you're watching closely :_> It always did the same thing. Other cars are worse, only there's no easy way to see it.

    The best way to warm up a Prius is the same as with any other car: drive it, gently for the first minute. Warming it up in the driveway is a waste of fuel unless you need to do it for comfort or safety (for example, defrosting).