Hand calculation vs mfd mpg - way off

Discussion in 'Gen 2 Prius Fuel Economy' started by gptoyz, Mar 1, 2019.

  1. gptoyz

    gptoyz Junior Member

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    So my mileage in my latest fill up was 311 miles on 5.5 gallons of gas which works out to 56.5 mpg

    My mfd is displaying 38.9 mpg.

    My fill up procedure is fill to the top with the trigger in the normal position until it automatically stops. Then I invert the gas pump and fill till it stops once again.

    My hand calculation procedure is to take my fill up amount and divide that into my trip range.

    I'm totally happy with 56.6 mpg heck I would be happy with 47 mpg! But I wish the mfd would be more congruent with my hand calculation.

    Anyway to calibrate the damn mfd calculator?


    iPhone ?
     
  2. bisco

    bisco cookie crumbler

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    something is not right
     
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  3. Wayne

    Wayne Active Member

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    Wow, that's a pretty big difference (45%!)

    I tracked this for 12-1/2 years on my 2005. No more effort, it was just an extra feature of my mileage spreadsheet. Depending on how carefully I filled the tank (say not so full the time before, then super-full to the point of running into the overflow container the next time [or vice-versa]), I found a single tank could vary by as much as 20%. Those tanks were super-rare though, and the error was usually that far in the opposite direction on the next tank, confirming it was probably a fill error on my part.

    After 12-1/2 years the two sets of total gallons used were only 3% apart, so overall my MFD agreed with reality nicely (probably better than my short-term hand calculations).

    So I would look at a timeframe longer than 1 tank, and never assume you will get any kind of accuracy in your manual calculation if you refill after only a few pips. Do 80-90% whole tanks. However, if that was a full tank, then I'd keep an eye on it. You may actually have a malfunction there. That's a pretty big difference!
     
    #3 Wayne, Mar 1, 2019
    Last edited: Mar 1, 2019
  4. Salamander_King

    Salamander_King Senior Member

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    How many times have you done this? Is it always this much off? I don't know about Gen2, but for Gen3 and now with PRIME (without EV), the displayed mpg is always 5-8% better than hand calculated mpg. This deviation is quite constant. You may want to check again using odometer reading, not tripmeter.
     
  5. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    In North America, the Gen 2 had a bladder inside the metal tank to reduce emissions, it was fantastic at that.
    However, it greatly complicated filling the tank, calculating mileage, and knowing when to refill the tank. so owners hated it.

    If your tank never seems to fill I offer: open the gas cap as soon as you get to the pump, the more time to vent, the more gas you can add. Try to add gas in the heat of the day, the bladder is more flexible when warm. if it still won't add gas, try the nozzle upside down.

    The only effective way to determine your Mileage is to average tanks.

    After running out twice I offer this:
    If your mother in law is in the car, get gas with 3 pips.
    with your spouse in the car, 2 pips means you never hear about it later.
    With friends who do not mind carrying at least 3 gallons of gas before you attempt to restart, one pip is fine.
    If the pip ever blinks pull over NOW.
     
  6. gptoyz

    gptoyz Junior Member

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    I do this for every single tank I fill up. It was driving me nuts that the MFD display is so off. But I can easily make it from SF to LA (via 152, 5, Grapevine) and then run around in socal. Generally, if I want to tempt fate, I can easily get 420-450 miles per tank when the last bar is blinking and my fill for a 400+ mile tank is about 8.X+ gallons.

    As far as the odometer vs trip vs MFD, they are all in consensus. The only difference with the Trip is that it will give me resolution to the tenth of a mile.

    My prius C was also pretty optimistic but it wasn't so grossly off.
     
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  7. gptoyz

    gptoyz Junior Member

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    Ahhh yes, I forgot, when I fill up at costco, and I only do this for Costco because of the lines, I do open my gas cap as I'm queueing. If I ever fill up at gas stations, not named Costco, I never get the opportunity to do this.
     
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  8. bisco

    bisco cookie crumbler

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    so, we're in a gen2 forum, but your info says 2010
     
  9. gptoyz

    gptoyz Junior Member

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    I had a lease g3 lease before and a c but this is in reference to my 08 g2


    iPhone ?
     
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  10. bisco

    bisco cookie crumbler

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    gotcha. definitely a bladder issue
     
  11. Mendel Leisk

    Mendel Leisk Titanic Social Director

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    I think you'll find the more tanks you calculate the more it'll true up. Just two tanks would pretty much cancel out any pumping technique variations, three for sure would do it.
     
  12. gptoyz

    gptoyz Junior Member

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    Yes agreed it's not isolated to two tanks. I have been doing this process for about 7k miles




    iPhone ?
     
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