50% improvement in miles per tank, 10% loss in MPG

Discussion in 'Gen 2 Prius Main Forum' started by ZGG, Jan 14, 2020.

  1. ZGG

    ZGG New Member

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    My average miles per tank of gas has suddenly increased to about 290-300 miles from about 205-220 (winter). Considering it is winter and cold in the part of north NJ that i live, i find this very confusing. This has been going on for the past month. Also, My MPG has reduced from 48 ish (winter) to 45 ish quite drastically in the same period of time.

    I have a 2007 prius with 256,000 miles. new battery from the dealer in 2016 at 160,000 miles.

    I am used to my prius getting about 250 miles per tank in the summer before it starts flashing to fill up, and barely 200 miles in the winter. I am used to about 50 mpg in summer and 48.0 in winter. i cannot fathom why i am able to get so many miles out of my tank now than before, i always felt the amount of gas indicator went "early" and that i still had gas, so maybe this is corrected now?

    I do all my own work on my car, and am very sounds mechanically and with electronics, and with my observations with this "event" that is happening? Can any one give me any info about how MPG is calculated, how the gas tank sensor works, or if this phenomenon has been observed?

    Many thanks friends.
     
  2. ericspoor

    ericspoor Member

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    The gen2 has a bladder in the gas tank, so a full "tank" isn't a fixed # of gallons on any given fill-up. if the bladder expanded it would account for increased miles per tank with decreased mpg. Just a thought.
     
  3. cthindi

    cthindi Member

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    Still the mileage per tank is a bit too low. I live in CT and have 2005 prius with 365K Miles. I get about 400 Miles in Summer and 300 in winter per tank. Mileage drop you see is quite typical. I used to get 50/46. Now it is down to 46/41.
     
  4. ydpplqbd

    ydpplqbd Active Member

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    I have a 2006 Prius and I am also in northern New Jersey. I have been keeping meticulous fillup and mileage info for months now because when* the flashing blin (AKA pip) turns on is so variable. Additionally, I have also been focusing on the number miles since last fillup when the following occurs:

    i.) when first blin turns off (I call first blin to turn off blin#10 as it is the rightmost blin of the ten blins);

    ii.) the mileage-since-fillup when last blin (what I call blin#1 and is the leftmost blin) is only blin illuminated (but not flashing); and

    iii.) the mileage-since-fillup when last blin is only blin illuminated and BEGINS flashing.

    *- "when" meaning how many miles into the tank the flashing blin first turns on.

    In order to prevent evaporative system issues, I avoid attempting to force additional fuel into the tank after the pump first clicks off. However, some station's fueling nozzles click off earlier than other station's fueling nozzles. As a result, I try when possible to use the same fueling nozzle at the same gas stations as much as possible.

    On my last tank of fuel, the first blin turned off after 109 miles of driving described above as "i"). I have had instances where the the first blin turned off after only 30 miles. I attribute such a large difference in mileage for situation "i" to: a. short fill where pump nozzle clicked off early; and b. also to the fuel bladder being smaller when it is colder outside.

    I have attempted to rely on the flashing blin in order to know when to fuel up. However, I have NOT felt confident in the flashing blin to rely upon it in order to "know" when to fuel up. Instead, I use 95% of projected range which I calculate as follows (last fill up number of gallons x average mpg last 5 fillups x 95%). My calculation for last night's fillup of 7.3 gallons is as follows: projected range ("PR")= 7.3gal x 48mpg= 350.4miles >>>> PR x 95%= 332.88miles. So, I will refuel next time at about 333 miles (give or take a mile or two). I have never been stranded or run out of fuel using this method. The only scary part of this method is that the last blin will likely be the only illuminated blin after 270 miles (situation "ii"). And, the blin will likely start flashing at 300 miles (situation "iii"). Which means that I will need to drive my Prius for about 33 miles with the last blin flashing. Scary initially but works nicely so long as you keep meticulous mileage and fueling records. Also, do not use this method if you are experiencing a check-engine-light situation, or HV battery death spiral as these are likely to reduce your mileage and thereby make the average mpg for the last 5 fillups irrelevant (and thus cannot be relied upon).

    Just my two cents ("$0.02").

    PS Variable fuel bladder size is interesting. But gasoline is essentially a non-compressible fluid thus if you pump in 7.5 gallons into the tank/bladder then you still have 7.5 gallons even if the weather becomes bitterly cold in a short period of time. As a result, I focus on gallons pumped and average mpg in order to keep track of when to fuel up.
     
  5. valde3

    valde3 Senior Member

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    Have you disconnected the 12V battery? That could (maybe) reduce the fuel economy and cause the fuel sensor calibration to go off.

    Have you calculated the fuel economy from fill ups? Over multiple fill ups? If you're just looking at the MPG in MFD then one thing that you could check is fuel trim numbers. They would indicate if you have some problem with fuel system.

    If it is two separate issues they are reduced fuel economy and gas tank now reading more when nearing empty. For fuel economy there's lots of talk here. For gas gauge it could be anything like sensor or calibration. You can do the calibration yourself.

    Only one problem that could cause both of these would be with fuel tank. It could reduce your fuel economy if engine does get some gas (vapor) though EVAP system. In a North American Prius that has bladder tank that would need a bad charcoal canister or a bad tank bladder. Pumping a tank too full might have cause either one of those. Either one of those could cause fuel to get outside of fuel bladder. You could maybe also et fuel outside of bladder by just hard overfilling the tank. Fuel on outside of the bladder would cause fuel gauge to read more as the bladder would raise and thus the level of fuel in it would raise. Unfortunate there's no easy way of knowing. If this is the problem you would now run out of fuel around the time when last pip starts blinking as you probably didn't have 80...85 miles of fuel left when you filled up in the past? How long have you driven with the last pip flashing? In the past? Now? But don't run out of fuel on purpose to test this. It's not a good idea.

    If there's no fuel on the outside of bladder then you have two separate issues.
     
    #5 valde3, Jan 16, 2020
    Last edited: Jan 16, 2020
  6. fragglestickcar

    fragglestickcar Junior Member

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    The tank capacity rises 50% but mpg falls 10%... sounds like nothing about the car's telemetry can be trusted.

    Started a measurement thread.
     
  7. dolj

    dolj Senior Member

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    I'm really curious as to where the term "blin" you use came from. It has always been called a pip everywhere else on PriusChat.
     
  8. ydpplqbd

    ydpplqbd Active Member

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    I used to work at a car dealership many years ago (mid 80s). IIRC that is what we called the fuel dots/blocks there. So that there is no confusion I usually reference the "blin" as also being known as a "pip."
     
  9. dolj

    dolj Senior Member

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    Somehow I don't think it will catch on. At least blocks or dots are real words if you don't like to use pips.
     
  10. ydpplqbd

    ydpplqbd Active Member

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    A rose is a rose by any other name.

    PS I am not trying to get "blin" to catch on. Just trying to communicate.
     
  11. dolj

    dolj Senior Member

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    Wouldn't you communicate better by using the accepted terminology?
     
  12. ydpplqbd

    ydpplqbd Active Member

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    "AKA pip" demonstrates that there are alternative words for the same item. Additionally, context also demonstrates what a "blin" is.
     
  13. ice9

    ice9 Active Member

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    As others have indicated here, it sounds like your increase in miles per tank has been cause by an expanded tank. Expect that this is permanent. It will probably remain near 300 even during the summer months. The decrease in mpg during the winter months is normal for a Prius.
     
  14. dolj

    dolj Senior Member

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    I would have thought it was normal for all cars, not just the Prius. Maybe it is that Prius owners are more sensitive to changes in fuel economy/consumption, given that, for some, it becomes a focus.
     
    jerrymildred likes this.
  15. cthindi

    cthindi Member

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    I feel both are true. All cars loose MPG since the engine and exhaust temperature remains the same all year. Thus cold winter air needs to be warmed up using fuel.

    May be Prius looses a fraction more in % fuel consumption since it prioritizes use of engine till it warms up. And of course Prius owners are more sensitive as a group to drop in MPG.
     
  16. ice9

    ice9 Active Member

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    Yes. And it's more noticeable in a Prius because owners have an instrument display that allows them to impress friends with their hypermiling prowess, or disappoints when it fails to meet expectations.
     
  17. sam spade 2

    sam spade 2 Senior Member

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    "Miles per tank" is a meaningless measurement.
    Except that it might indicate that your tank is not filling completely........for whatever reason.
     
  18. chelvis

    chelvis Member

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    Another North Jerseyan here weighing in. I bought my 2005 brand new and now have 319K miles on the car. I have cursed that gas tank bladder for decades now, but have been granted serenity to accept its limitations. Those miles per tank are rather low, something else odd is happening there. Happy motoring to all!
     
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