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At what temperatures can I expect gas bladder silliness?

Discussion in 'Gen 2 Prius Fuel Economy' started by brightlights, Sep 10, 2016.

  1. brightlights

    brightlights Junior Member

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    TL;DR: My arithmetic MPG is 62, my MFD is 55.4, and I am skeptical. At an average daily ambient temp of 85 degrees in the last week (compared to 72 when I last filled up,) how much can I trust that the quantity needed to "fill" the gas bladder actually filled it up?

    Long version, with answers to question list in quotes at bottom:

    91k 2008 model, I've had it for two and a half weeks so I'm still learning. Drove it home from the dealer with less than half a tank, and when I filled up, put in 8.4 gallons. My first full tank, went 390 miles and again filled up with 8.4 gallons. Arithmetic average was 47 MPG (I hadn't reset the MFD display for this tank.) I filled up the tank when it was about 73 degrees outside. I then drove another 390 miles, over which the MFD gave me an average 55.5 MPG. The gas pump clicked off at 6.262 gallons, which the gauge read as full. This gives me an arithmetic average of 62 MPG. The last week has been quite warm with daily highs around 95 and nights in the low 70's.

    I would expect my arithmetic average to be lower than the MFD- I understand the display to be optimistic. That said, I also wouldn't expect the bladder to be constricted at these temperatures. As I'm learning the car, I'm trying to get a good feel for my actual MPG, not only to learn to drive more efficiently but also to be able to make sensible decisions about when to fuel up come wintertime when I can expect the gas bladder to be constricted and the gauge to be questionable. What are the odds that that 6.2 gallons I put in today actually filled me up, or to put this another way, am I right to be skeptical of the arithmetic average?

     
  2. wjtracy

    wjtracy Senior Member

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    8.4 gal is typical for summer...this is as good as it gets...you may find some pumps kick off earlier. The way I can tell how well I filled up, there is about 150 miles @50MPG before the first pip goes away on a good fill up. In winter you may find 6-7 gals is all the bladder lets in, and your MPG will be lower, and the first pip goes away a lot sooner - and you could be 300 or under miles per tank. Your hand calc will always scatter due to variations when the pump clicks off earlier or later, so on the hand calc you always need to average at least several tanks.

    PS-welcome!
     
  3. tvpierce

    tvpierce Senior Member

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    As you know, the bladder can screw with your calculations on a single tank. But if you calculate over multiple tanks, you can virtually eliminate the bladder as a variable. Here's the key though: you have to calculate total miles driven by total fuel used over multiple tanks. If you use your calculated MPG from each tank, you'll skew your average.
     
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  4. Mendel Leisk

    Mendel Leisk Senior Member

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    ^ What he said. Even the accumulated mpg of 2 tanks should dismiss most of the tank-to-tank disparities. More tanks, it gets ever more deadly.

    That said, it's still frustrating, it's good to know how you're doing on individual tanks.
     
  5. Lucifer

    Lucifer Senior Member

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    Had a gen2, to answer your question, around 45-50 degrees the bladder would start to contract(when the temp is dropping), so living in NH, the spring and fall would bring days of temps that made the bladder difficult to gauge, but it's not something to dwell on, a gallon at most is concerned.
     
  6. bisco

    bisco cookie crumbler

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    i never had a bladder issue in my 2004 or 2008, withe temps ranging from zero to 100. some cars seem worse than others for some reason, so i can't say for for sure if or when you will have a problem.

    as for you current mpg's, you need to log a few tankfuls and start averaging them together for an accurate reading.
     
  7. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I can report gas bladder silliness between 18 F and 105F. Try to focus on what a good job it is doing reducing pollution.
     
  8. brightlights

    brightlights Junior Member

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    Thanks for all the responses. So if I'm synthesizing this correctly:
    1. The gas gauge and gas bladder are both unreliable witnesses. Therefore, even in warm conditions, they are not to be relied upon for calculating MPG of a single tank.
    2. If I would like to estimate MPG of a single tank, even though the MFD is not fully accurate, it may (is likely to?) be more accurate than the arithmetic MPG, because of #1.
    3. Because of #1 and #2, I better wait at least a month before replacing my tires so I have a more longitudinal calculation that will let me figure how much I'm losing by switching... or then again, maybe I don't want to know. :)
    The statistician in me is crying, but I knew about the questionable accuracy factors coming in, and if I can't take a joke I shouldn't have joined! (Also, by any measure I'm getting at least 50 MPG mostly highway, probably more than that, and therefore I have nothing to complain about.)

    Bingo!

    Thanks!
     
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  9. bisco

    bisco cookie crumbler

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    your car reading is probably a few miles north of actual, and most likely as good as you get until you do long term testing. as far as tyres go, why look back? if you want mpg's, get a proven winner. if you want something else, you may have to sacrifice a few mpg's.
     
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  10. brightlights

    brightlights Junior Member

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    Right, I'm well aware that by switching to an all-season tire in comparison to the Integrities currently on board, I'm going to lose MPG- but gain substantial safety this winter. I'm very familiar with the difference in handling a stout all-season tire can bring around here and I'd like to be able to drive up my road come winter!
     
  11. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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  12. bisco

    bisco cookie crumbler

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    do you get a lot of ice and snow in balimore?
     
  13. brightlights

    brightlights Junior Member

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    I see you're in Boston, so you laugh in the face of our weather! The last few years we've had at least one storm with several feet of snow but consistent volume doesn't tend to be a problem. Road clearance and collective driving skill are the bigger issues- although I live 5 minutes from I-95, my commute also includes secondary highways and residential streets which are often questionably plowed, and there's a running joke that Baltimore drivers forget how to operate their vehicles as soon as it starts to precipitate. In general, ice and wet traction are more important as I'm more likely to see slush with ice patches than to have any compelling reason to try to emulate a 4WD vehicle (read: "be stupid on a public road.") I do need to be able to drive confidently in tropical storm-like conditions as I see those pretty regularly during the summer.

    I've never felt the need to run snow tires but all-seasons do make a difference. On my Civic I used Goodyear Assurance TripleTreds and that was all I needed, including driving uphill through several inches of snow cover. Great tires- sorry they don't come in an appropriate size for the Prius' stock wheels. I'm starting to look now at Continental TrueContact. I see those have gotten good reviews from the group here. I imagine you need a proper snow tire in your neck of the woods?
     
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  14. bisco

    bisco cookie crumbler

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    one good all season low rolling resistance tyre is michelin energy saver.
     
  15. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    [QUOTE="brightlights, post: 2418277, member: 147841']The statistician in me is crying, but I knew about the questionable accuracy factors coming in, and if I can't take a joke I shouldn't have joined! (Also, by any measure I'm getting at least 50 MPG mostly highway, probably more than that, and therefore I have nothing to complain about.)[/QUOTE]
    I suspect North American Gen 2 owners are over represented on Fuelly, as it provides us with analysis we need so much more than other car owners