How to calculate MPG?

Discussion in 'Prime Fuel Economy & EV Range' started by Mooney Driver, Jul 29, 2021.

  1. Mooney Driver

    Mooney Driver New Member

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    I am a bit confused as to how to calculate MPG with a Prime. When it's time to gas it up do you divide total miles between fill-ups by the # of gallons to fill tank? Is this how Toyota comes up with the figure they display on the sticker?

    What are you finding to be the MPG you're getting with your Prime?
     
  2. Salamander_King

    Salamander_King Senior Member

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    If you do not drive the PP on EV mode (i.e. no charging from the wall), then your method will give you accurate mpg. But if you drive any portion of the trip with EV mode using the traction battery charged from the wall, then your mpg is not accurate fuel efficiency of the engine-driven part (i.e. HV mode drive).

    EPA 54mpg is on pure HV mode without EV range included. PP has no display that distinguishes the miles run on EV mode or HV mode, thus you can't easily calculate the HV mode mpg from that. Some people who do mostly EV drive gets above 500mpg on display as well as by calculating your way. Theoretically, if your trips are always within EV range and you never need to start the engine for other reasons, the car will drive forever without ever using a once of gas, which will give you 999.9mpg on the display and the calculated mpg will be infinity (miles driven divided by zero). You can try driving without charging your car for a full tank to get an idea of what mpg you are getting, but if you regularly mix EV mode on your full tank like most PP owners, then I found using DTE after filling a tank is the reasonable solution to estimate the mpg of HV mode portion from the last full tank.

    After a fill-up, check the DTE distance displayed. Then divide that number by 9.3. That will give you an estimated mpg from the last tank.

    For example, if the display is 540miles, then 540/9.3=58.1mpg. If 450miles of DTE is at full, then 450/9.3=48.4mpg. The key is you always divide the DTE miles by "9.3". This is the amount of gas used from a full tank to the empty light that comes on, at least that's what I found on my previous 2017 PP. For accuracy and consistency, try to fill at the same gas station using the same pump and insert nozzle same depth and stop filling at the automatic stop without doing any top off. Then try to drive the car until the empty light comes on and then re-fill again. You can use EV mode as much as you want, but more EV mode, the longer it will take to get to the empty tank.

    I don't always follow those procedures, but I can get within 5% of the deviation from the estimated mpg based on the miles driven on the EV and HV portions.

    See this thread for more discussion.
    What is your DTE after fill-up? | PriusChat

    This my comment summarizes the result of 5 fill-ups in a spreadsheat.What is your DTE after fill-up? | Page 6 | PriusChat
     
    #2 Salamander_King, Jul 30, 2021
    Last edited: Jul 30, 2021
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  3. Mooney Driver

    Mooney Driver New Member

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    thanks for a really comprehensive response.
     
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  4. Mendel Leisk

    Mendel Leisk Sand Pounder

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    MPG stands for Miles Per Gallon. Divide the miles you've driven, by the gallons you’ve used.
     
  5. Salamander_King

    Salamander_King Senior Member

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    Yeah, but on PP, it means miles driven on both electricity and gas. Not very useful information as displayed on the car or as calculated regular way.
     
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  6. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Ah, good point. Yeah my method would mean driving a tank or two without charging.
     
  7. Salamander_King

    Salamander_King Senior Member

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    Yep, that was what I did from time to time just to get the baseline mpg on HV only. But for many PP owners, not charging the car every day and burning a full tank of gas just to get the mpg information is an unthinkable sin. For those people, the DTE method I mentioned above works well enough within 5% of the more detailed method (recording amount of kWh used for the daily traction battery charging and calculating EV miles driven on the full tank distance).
     
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  8. Mooney Driver

    Mooney Driver New Member

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    What if you recorded the DTE PRIOR to fill-up and then subtracted it from the DTE AFTER fill-up. You then take this number and divide it by the gallons pumped to fill the tank. Wouldn't this number provide an accurate MPG between fill-ups?

    In other words: (DTE after fill-up - DTE before fill-up)/ total gallons to fill tank= MPG
     
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  9. Salamander_King

    Salamander_King Senior Member

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    Yeah, that method has been suggested by others, but I have never recorded the DTE number before the fill-up, so I don't have any data to comment on the accuracy of your method.

    But AFAIK, the DTE estimates the "distance to empty" based on the past performance, most likely since the last fill-up but it could be a longer period than that. DTE has no predictive power on future performance. My method is simple enough because I always used DTE miles after fill/9.3. 9.3 gallons being the gallons used from a full tank to when the empty light first comes on which should correspond to the mile when DTE reaches --mile.

    Here is the spreadsheet I posted in my other thread:
    Note the boxed number from the full tank drive I did purely on HV mode. The distance was 473miles, and it took 8.47 gal at fill-up. The displayed mpg for this trip was 59.5mpg while the full-tank method calculated was 55.86mpg. DTE immediately after fill-up was 538miles, divide this by 9.3, my DTE method mpg was 57.84.mpg. They are all close enough.


    upload_2021-7-30_10-51-13.png
     
    #9 Salamander_King, Jul 30, 2021
    Last edited: Jul 30, 2021
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  10. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Wish list for Toyota, for the Prime displays:

    1. Show EV percent
    2. Show MPG with EV portion ignored
    3. And I know it’s not in their DNA to do this: quit “cooking the books”, on the MPG displayed
     
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  11. Salamander_King

    Salamander_King Senior Member

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    PP actually shows EV percent. But it includes the EV portion driven miles under HV mode. It is quite useful if one just wants to figure out the miles the engine was running. But not very useful if one wants to calculate the pure HV only mpg.

    What Toyota is doing is to calculate the total miles driven and the sensor is estimating the amount of gas used when the engine is on to get the mpg. Unfortunately, if they try to ignore the EV mode driven EV miles, then it may also ignore the EV miles driven under the HV mode. Sure there must be ways to distinguish EV mode miles driven from the charge from wall vs EV drive portion of HV mode driven by primarily regen energy stored in the battery. But both are using 100 electricity to propel the car, so there is no difference in the current sensor mechanism.
     
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  12. jerrymildred

    jerrymildred Senior Member

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    Since there is an indicator on the dash that comes on in EV mode, the car knows and if it knows, it can track and display EV mode miles and HV mode miles independently if they'd take the time to program it and make a place to display it.
     
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  13. Ovation

    Ovation Junior Member

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    Using the DTE/9.3 method on my second to last fill-up, I get (after metric to US conversions) 62.933333333 MPG, which I would lazily round down to 60 MPG.

    My most recent fill up (90% autoroute driving at around 115-120 km/h) yields (after conversions) 57.866666667 MPG, which I would lazily round down to 55 MPG.

    I know there’s more room for precision but it’s close enough for me and spectacularly better than the 15.175134406113 MPG I rigorously tracked with my Xterra over a 7 year period.

    When I factor in the EV mode, effective MPG frequently ranges from 68-73 MPG. Can’t possibly complain about that (especially with my 1600 km weekly commute this semester).
     
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  14. Salamander_King

    Salamander_King Senior Member

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    Did the first long trip on my new 2021 PP yesterday. Started with a first fill-up of the tank after the purchase. Total 549.1 miles of which ~35miles was on EV, the rest all on HV entirely on the interstate highway at ~70mph, and still have 80 miles left on the DTE. Displayed mpg for the day 64.4 is going to be a bit higher than the actual number, but at this speed and three people in the car with fully loaded cargo, it is going to be at least double the figure I get with our PathHy. But, I have learned, for this length of driving, the PathHy seat is much more comfortable than PP.

    upload_2021-8-29_18-39-36.png
     
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