avg mpg accurate, single drives not so much

Discussion in 'Gen 4 Prius Fuel Economy' started by nema32, Aug 20, 2022.

Tags:
  1. nema32

    nema32 New Member

    Joined:
    Aug 20, 2022
    4
    2
    0
    Location:
    Albuquerque NM
    Vehicle:
    2022 Prius
    Model:
    N/A
    I'm trying to understand why my avg mpg is correct (verified by odometer / gal), but individual drives aren't.

    I'm getting 76.0 avg mpg right now over 1570 miles. I drive almost entirely to work and home. The path is mostly downhill one direction, mostly uphill the other. One direction is usually 150-190mpg listed when I turn off. The opposite direction is usually 50-60mpg. (8.7 miles each direction, 17.4 daily)

    Back of the envelope yields: (170 + 55) / 2 = 112.5. That's not even close to 76.0. What's going on?
     
  2. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    99,186
    44,937
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    welcome!

    the average mpg on the dash is 76, and computed at the pump is also 76?
     
  3. nema32

    nema32 New Member

    Joined:
    Aug 20, 2022
    4
    2
    0
    Location:
    Albuquerque NM
    Vehicle:
    2022 Prius
    Model:
    N/A
    yeah, they are in agreement. I'm just trying to figure out how the individual driving sessions can average much higher when that's obviously not the case.
     
    bisco likes this.
  4. Doug McC

    Doug McC Member

    Joined:
    Feb 15, 2022
    307
    115
    0
    Location:
    Northwest Illinois
    Vehicle:
    2022 Prius
    Model:
    XLE
    Take a few minutes to dive deep into your OM. It actually explains why you are seeing what you’re seeing. Also, get ready for some REALLY IGNORANT responses! ;). I’ve got a’22 also, and see very similar averages. Again, the manual actually addresses it and it actually makes sense.
    Some here believe (and remember, beliefs are seldom based on real world facts anymore) that Toyota has something to gain by convincing you that you are getting better mpg than you are (apparently believing, also that Toyota thinks none of us can do basic math), so get ready for the conspiracy advocates to chime in on this: trust your math and your manual. ;) ;)
     
    #4 Doug McC, Aug 20, 2022
    Last edited: Aug 20, 2022
  5. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    16,009
    9,200
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    You are using the wrong math to compute combined trip average, because the denominators don't match. Try this instead:

    Downhill fuel used: 1 / 170 gallons per mile = 0.005882 gal/mile
    Uphill fuel used: 1 / 55 gallons per mile = 0.018182 gal/mile
    Round trip sum = 0.005882 + 0.018182 = 0.024064 gallons per 2 miles round trip
    Round trip MPG = 2 miles / 0.024064 gallons = 83.1 MPG.

    83 is much closer to your displayed 76, than was the method you used. And considering that you gave round numbers, that is close enough.

    Most countries express fuel economy as liters per 100 kilometers. If we used either that, or an English equivalent such as gallons per 100 miles, then we could do a simple average and get the right answer.
     
    #5 fuzzy1, Aug 20, 2022
    Last edited: Aug 20, 2022
    CR94, bisco and alanclarkeau like this.
  6. nema32

    nema32 New Member

    Joined:
    Aug 20, 2022
    4
    2
    0
    Location:
    Albuquerque NM
    Vehicle:
    2022 Prius
    Model:
    N/A
    Ugh. Thanks for taking time to explain that error. I'm glad it was that simple and that it more or less matches using the right method. It's just embarrassing b/c I did actually pass diff-eq a few decades back.
     
    fuzzy1 likes this.
  7. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    6,992
    7,525
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    The other thing I recall from High School mathematics - you can't average averages (unless - as mentioned above, the denominators are identical, or at least similar).
     
    Doug McC likes this.
  8. Mendel Leisk

    Mendel Leisk Underfoot

    Joined:
    Oct 17, 2010
    48,216
    34,438
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    Oh geez, I've got to rise to that:

    upload_2022-8-21_9-45-55.png
     
    Doug McC, bisco and fuzzy1 like this.
  9. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    16,009
    9,200
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    My own diff-eq and complex variables and linear algebra are also extremely rusty by now.
     
    alanclarkeau likes this.
  10. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    6,992
    7,525
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    ditto :oops: . I did schooling BC (before calculators). Even slide-rules were ... I think steam driven.

    But - since school - I haven't knowingly used calculus, differential equations - or most of the trigonometry I learned. BUT - the learned concepts are undoubtedly in use by my brain still, 50 years later.
     
    Doug McC and fuzzy1 like this.
  11. Doug McC

    Doug McC Member

    Joined:
    Feb 15, 2022
    307
    115
    0
    Location:
    Northwest Illinois
    Vehicle:
    2022 Prius
    Model:
    XLE
    LOL I wondered how long you could let me get by with that! ;)
     
    Mendel Leisk likes this.
  12. Mendel Leisk

    Mendel Leisk Underfoot

    Joined:
    Oct 17, 2010
    48,216
    34,438
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    It's funny, looking back on the stuff we were taught:

    Penmanship
    Arithmetic
    Geography
    History (ancient history)

    Today's grade school curriculum pale. Maybe for the best.
     
  13. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    6,992
    7,525
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    Sometimes I feel like I came from there - the way things are progressing.

    All my cars in the past - you press the button to open the glovebox.

    A friend was telling me that he had to ask how to on his new car:

    Answer: To open the glove box:
    1) tap the car icon (Controls) at the corner of the screen
    2) tap the Glove box button on the screen
    .
    3) The glove box opens.
    4) you roll your eyes just wondering.
     
    Mendel Leisk likes this.
  14. CooCooCaChoo

    CooCooCaChoo Active Member

    Joined:
    Jan 7, 2015
    747
    420
    0
    Location:
    Cow-lee-fornia
    Vehicle:
    2017 Prius
    Model:
    Two Eco
    Soon, you will have to buy a subscription to use your glovebox. Thanks BMW!
     
    Doug McC and alanclarkeau like this.
  15. CR94

    CR94 Senior Member

    Joined:
    Dec 2, 2014
    1,967
    799
    0
    Location:
    Northwestern S.C.
    Vehicle:
    2011 Prius
    Model:
    Two
    Yeah, lots of people have trouble with basic arithmetic. Gallons per mile would be less confusing than miles per gallon. Average mpg is the inverse of the average of the inverses. Thus
    (1/170+1/55)gal./mile/2 = (1/83.1)gal./mile .
     
  16. CooCooCaChoo

    CooCooCaChoo Active Member

    Joined:
    Jan 7, 2015
    747
    420
    0
    Location:
    Cow-lee-fornia
    Vehicle:
    2017 Prius
    Model:
    Two Eco
    ^^^ You just made it more confusing!
     
    Mendel Leisk and fuzzy1 like this.
  17. ColoradoBoo

    ColoradoBoo Active Member

    Joined:
    Jul 19, 2019
    320
    230
    3
    Location:
    Monument, Colorado USA
    Vehicle:
    2017 Prius
    Model:
    Two
    While most say their dash MPG is under what they actually get, mine is consistently 2-3 mpg less. (Says 59-60 but when I fill it up it's 57-58) I suspect it due to being at 7,000 feet above sea level and our regular gas is 85 octane.)

    Recently, however, I replaced my engine air filter and inflated the tires to 37 mpg (recent cold caused TPMS light to come on and few were low.) Now, the car is reporting 64 mpg so it'll be interesting to see if I actually get 61-62 now.
     
  18. Mendel Leisk

    Mendel Leisk Underfoot

    Joined:
    Oct 17, 2010
    48,216
    34,438
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    At least for me dash-displayed mpg is invariably higher than calculated, by an average ~7.5%. I think that’s the consensus here too?
     
    ColoradoBoo likes this.
  19. fuzzy1

    fuzzy1 Senior Member

    Joined:
    Feb 26, 2009
    16,009
    9,200
    90
    Location:
    Western Washington
    Vehicle:
    2012 Prius
    Model:
    Three
    They do? That is the opposite of what I've been reading. Like yours, my dashboard MPG is a bit higher than reality. The vast majority of reports here, at least on my vehicle generation, are similar.
     
    ColoradoBoo likes this.
  20. Ryder99

    Ryder99 Junior Member

    Joined:
    Feb 9, 2022
    38
    32
    0
    Location:
    S.E. PA
    Vehicle:
    2022 Prius
    Model:
    XLE
    My dash reports better mpg most of the time. Sometimes it is correct.

    10k @ 52.9 mpg
     
Loading...