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

Keeping Accurate MPG Fuel Logs-Help!

Discussion in 'Gen 2 Prius Fuel Economy' started by Matt Herring, May 4, 2009.

  1. Matt Herring

    Matt Herring New Member

    Joined:
    Apr 28, 2009
    421
    51
    6
    Location:
    North Andover, MA
    Vehicle:
    2010 Prius
    Model:
    II
    Obviously, tracking mpg in a 2009 Prius with actual miles driven divided by gallons filled is not accurate given the bladder in the gas tank. I'm curious to know how others accurately estimate their per tank fuel mpg.

    I have a SGII and despite improving from 54.4 to 58.5 mpg from my first tank to my last tank my actual fill mpg dropped 1 mpg. My in-car mpg went from 57.9 to 62.0 mpg between these two tanks as well. Actual fill mpg dropped from 56.5 to 55.45 mpg.

    Does anyone with a SGII have a percent optimistic/pessimistic they use to calculate generally accurate mpg (preferable someone with a decent number of tanks under their belt)? Or does anyone have a percent optimistic/pessimistic they use from their in-car mpg? I have heard the in-car mpg guage runs about 1 mpg optimistic.

    I keep detailed, accurate fuel logs for my vehicles and I would like to zero in on my results. Thanks!
     
    #1 Matt Herring, May 4, 2009
  2. efusco

    efusco Moderator Emeritus
    Staff Member

    Joined:
    Nov 26, 2003
    19,891
    1,191
    9
    Location:
    Nixa, MO
    Vehicle:
    2004 Prius
    Model:
    N/A
    Tracking by miles driven divided by gallons used is, by far, the most accurate way to record your mileage over time. On a tank by tank basis using the MFD MPG (if you hit reset on the '06+) or via Scan Gauge II is more accurate b/c of variability of filling capacity.
     
    #2 efusco, May 4, 2009
  3. Matt Herring

    Matt Herring New Member

    Joined:
    Apr 28, 2009
    421
    51
    6
    Location:
    North Andover, MA
    Vehicle:
    2010 Prius
    Model:
    II
    I have kept detailed fuel logs on my vehicles for a very long time and realize they are paramount to tracking my car's success but the Prius fuel bladder does not help in this regard...no matter how many entries I make (if I posted 62.0 in-car and 58.5 SGII but 55.45 actual fill...I could do this endlessly and still not get acurate mpg on my fill totals).

    So...I'm asking more veteran Prius fuel loggers what their MFD and SGII has read for them over time...and what I should adjust each to better estimate my mpg.

    efusco...what's your in-car and SGII over time that you adjust to to get accurate mpg?

    I'd like to have a good representation of my actual consumption mpg numbers without being too far off when I do my entries.

    1st tank in-car MFD mpg = 57.9 mpg
    2nd tank in-car MFD mpg = 62.0 mpg

    Can you accurately estimate my actual mpg based on these tanks usin the knowledge you have gained from numerous tanks?
     
    #3 Matt Herring, May 4, 2009
  4. efusco

    efusco Moderator Emeritus
    Staff Member

    Joined:
    Nov 26, 2003
    19,891
    1,191
    9
    Location:
    Nixa, MO
    Vehicle:
    2004 Prius
    Model:
    N/A
    I use a CAN-View, not Scan-Gauge, but don't use it to track long term, I use calculations.
    But on average I find that the MFD tracks ~2% higher MPG than my calculated.
     
    #4 efusco, May 4, 2009
  5. Matt Herring

    Matt Herring New Member

    Joined:
    Apr 28, 2009
    421
    51
    6
    Location:
    North Andover, MA
    Vehicle:
    2010 Prius
    Model:
    II
    Thanks in advance for your help!
     
    #5 Matt Herring, May 4, 2009
  6. Matt Herring

    Matt Herring New Member

    Joined:
    Apr 28, 2009
    421
    51
    6
    Location:
    North Andover, MA
    Vehicle:
    2010 Prius
    Model:
    II
    Have you seen that 2% higher mpg move up or down significantly or is this a good number to use when entering my fuel logs?

    Thanks for all the help...I'm a new Prius owner and heavy into tracking and logging my usage...just need good direction to accurately record my mpg tank to tank.
     
    #6 Matt Herring, May 4, 2009
  7. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
    Staff Member

    Joined:
    May 11, 2004
    14,816
    2,497
    66
    Location:
    Far-North Chicagoland
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Advanced
    I agree with Evan. The only way to accomplish an accurate mileage history is to track total miles and total gallons. On a tank-by-tank basis, I base my numbers exclusively on the MFD reading.

    To see my mileage tracking spreadsheet, click the link in my signature.
     
    #7 TonyPSchaefer, May 4, 2009
  8. Matt Herring

    Matt Herring New Member

    Joined:
    Apr 28, 2009
    421
    51
    6
    Location:
    North Andover, MA
    Vehicle:
    2010 Prius
    Model:
    II
    Nice fuel log Tony!!!:rockon:

    I also agree that an accurate mileage history is the way to go but if you're using in-car MFD mpg are you also adjusting other info. to reach that number since actual miles driven divided by actual gallons fill are always going to be off compared to your in-car MFD mpg?

    In my case I had 62.0 mpg in-car MFD but my actual fill totals were 55.45 mpg. If I'm tracking in-car MFD in my log I would have to adjust miles driven or gallons used to make that work.

    What are you adjusting in your fuel log to equal in-car MFD mpg or is your fuel log your actual fill and miles driven?
     
    #8 Matt Herring, May 4, 2009
  9. JimboK

    JimboK One owner, low mileage

    Joined:
    May 1, 2006
    2,817
    187
    49
    Location:
    Chesterfield, VA
    Vehicle:
    2005 Prius
    You might find this helpful, where several of us have documented our mileage for several tanks, along with our documentation methods and spreadsheets. Mine makes an adjustment based on miles driven.
     
    #9 JimboK, May 5, 2009
  10. john1701a

    john1701a Prius Guru

    Joined:
    Jan 6, 2004
    12,749
    5,243
    57
    Location:
    Minnesota
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Advanced
    Actual fill and miles driven are the only way to go. Though, I do note (for my 8.5 years of data) what was displayed on the webpages & spreadsheets
    .
     
    #10 john1701a, May 5, 2009
  11. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
    Staff Member

    Joined:
    May 11, 2004
    14,816
    2,497
    66
    Location:
    Far-North Chicagoland
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Advanced
    I am not adjusting anything. This is not a modification to mathematics, it's a paradigm shift. It is not valid to compare the miles/gallon calculation to the MFD reading. I know that the Prius tracks the number of miles driven and it knows how many times it squirts fuel into the cylinder. It knows these things much better than I do.

    You already acknowledged the bladder and the variation in capacity it creates but have not yet mentally allowed yourself to stop doing the miles/gallon calculations on a tank-by-tank basis.
     
    #11 TonyPSchaefer, May 5, 2009
  12. Matt Herring

    Matt Herring New Member

    Joined:
    Apr 28, 2009
    421
    51
    6
    Location:
    North Andover, MA
    Vehicle:
    2010 Prius
    Model:
    II
    Got ya...just different from other cars I have driven and was only looking for more veteran Prius drivers to give me a good model to use when tracking mpg. Set up my excel spreadsheet this morning and will continue tracking mpg using the examples provided.

    Thanks for the help!
     
    #12 Matt Herring, May 5, 2009
  13. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

    Joined:
    Apr 13, 2004
    15,140
    611
    0
    Location:
    South Puget Sound, WA
    Vehicle:
    2013 Nissan LEAF
    Model:
    Persona
    my sig posts info based on the MFD and the "paper and pencil" mode.

    the only data common to both is miles traveled.

    for instance, i drive 500 miles, MFD says 50.00 mpg and i put in 9.8 gallons...

    so paper and pencil is miles traveled (500) divided by gallons added (9.8) = 51 MPG

    MFD is miles traveled (500) divided by MPG (50) = estimated gallons used (10)....

    now to overcome bladder inconsistencies just track both totals over time. the more fillups you have, the lower the error due to inconsistent fillups over the year. notice that in winter, MFD will be consistently lower; in summer, higher.

    why is that? dont know. i do notice a slight difference in volume but not much. mostly goes with simply getting much better gas mileage in summer which allows me to push the end of the tank more. for a long time, i thought the bladder was larger in warmer weather, but not really thinking that anymore

    **edit**

    just to add, for accuracy, i use Trip A for miles traveled to get to the tenths. just remember to reset Trip A and the MFD on every fillup. also, i noticed; it takes a while for the Pri to register a new fillup and so, if you take off right away, i notice the diff bet Trip A and the MFD. now by difference i mean when the mile indicator changes. if i take off fast enough, i will see the MFD change a few seconds after the Trip A. now i have only seen one time when it was nearly a tenth off (iow, trip A is at mile.1 before or just after MFD goes to the same mile reading)
     
    #13 DaveinOlyWA, May 5, 2009
(You must log in or sign up to post here.)