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A Little Perplexed....

Discussion in 'Gen 2 Prius Fuel Economy' started by srellim234, Jun 25, 2017.

  1. srellim234

    srellim234 Senior Member

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    As you know, I recently had the 120k service done including replacing belts and the 12v battery. After that I performed the Prolong battery conditioning. I've been pleased with the first couple of tanks afterwards, 47 and 46 mpg, but yesterday left me scratching my head a little bit.

    I filled up at my regular ARCO station and headed to State Line (183 miles each way) to play golf. 72 mph almost all the way with the a/c on. Mileage screen showed 46.8 mpg when I got there. The car sat in an average of about 105* heat with the windows cracked, the rear curtain pulled and the reflective shield in the front window. It was between 113* and 120* all the way home. The screen dropped a bit, showing 45.8 for the round trip.

    It was down to two bars on the gas gauge so I went straight to my trusty ARCO station, even happened to get the same pump. Filled it up to the tune of a calculated 40.6 mpg! I've never had that big of a discrepancy from the screen before and that's worse actual mileage than I was getting in similar conditions before.

    Two things:

    1. Would that 5 hours sitting in that parking lot heat really have killed the mileage that much? If so, should it recover on its own?

    2. Is it most likely that the prolonged heat wave and then that extreme heat sitting in that lot made the gas tank bladder that much more pliable so it just accepted more fuel than it has been lately?

    I'm wondering what is most likely because I probably won't have the opportunity to run all the way through another tank this week and we're supposed to leave for the East Coast next Saturday.

    BTW, the car is in the garage with the windows open when here at home.
     
    #1 srellim234, Jun 25, 2017
    Last edited: Jun 25, 2017
  2. JC91006

    JC91006 Senior Member

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    Sounds normal to me (except you golf in 115 degree heat in Primm)

    Calculating mpg is like running a business, can't use 1 month of sales to determine the overall sales. Gotta do it over a 1 year period as conditions change throughout the year
     
  3. srellim234

    srellim234 Senior Member

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    I know, I have a book on all fills since we bought the car last year. It just threw me that there was that much of a discrepancy between the MFD and the calculated mileage. Which is also what led me to think the flexible bladder is the culprit. I mainly wanted to confirm that you guys also thought it was the bladder and not the battery so soon after the reconditioning.

    As for the golf, a friend and I play in Palm Springs and Laughlin all summer. We get to play some really nice resort courses for under $20 including riding carts and ice chests. The $12 Primm charged us yesterday includes bottles of water, GPS and free range balls at both the driving range and chipping practice area. The hospitality is great since they have to make sure the players staying at the Primm casinos who are paying through the nose for "stay & play" packages enjoy it and come back. Add to that the fact that hardly anyone ventures out in the heat and we have our own private courses with no one pushing us. The keys are staying well hydrated and parking the cart in the shade at every opportunity.
     
  4. jdenenberg

    jdenenberg EE Professor

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    My 2016 Prius does not have a "bladder" and the computer regularly overstates the MPG by about 4% (compared to a manually calculated MPG). My previous Prius (2004) had a bladder which does contribute high variability to the calculated MPG, but the long term average MPG was also overestimated by the computer by the same 4%.

    JeffD
     
  5. srellim234

    srellim234 Senior Member

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    Mine is routinely overstated by one or two mph but it's never been off by five before.
     
  6. JC91006

    JC91006 Senior Member

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    Filling up in high temperatures is never recommended as you get less fuel for your money. Something about the density of gasoline.....add that to the prius bladder and you'll get your discrepancy
     
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  7. jdenenberg

    jdenenberg EE Professor

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    4% of 52 MPG is only 2 MPG.

    JeffD
     
  8. jerrymildred

    jerrymildred Senior Member

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    My guess would be a really warm, stretchy fuel bladder. I wonder how great it will look next time if it's a cool day when you fill up. My '05 was all over the place!
     
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  9. JimN

    JimN Let the games begin!

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    My money is on the bladder and fueling discrepancies. Was any gas added after the automatic shutoff? Usually a phenomenally low tank and a phenomenally high tank followed each other. Averaging them together resulted in an average tank. It took two high tanks in a row to confirm a high tank.
     
  10. srellim234

    srellim234 Senior Member

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    No gas is ever added after the shutoff and I know true mileage is only realized over the course of many tanks but this was such an outlier it bothered me.
     
  11. Moving Right Along

    Moving Right Along Senior Member

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    I just looked at your Fuelly data, and the outlier is not your miles per gallon, but the amount of gasoline you were able to put in your tank. Your last 2 tanks before this one were around 6.6-6.8 gallons, but then this one was over 9 gallons. The larger amount of gas you could fit in your tank was probably due to the heat expanding the gasoline bladder, but your MPG for that fill-up is going to be deceiving because you put so much more gas in your car than you had before. On your next fill-up, you will more than likely see outstanding MPG since the car has 9.4 gallons to work with and it will probably get less than that amount of gasoline (unless the temperature is just as high). I saw the same kind of MPG bounce in my car in the last month.
     
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  12. srellim234

    srellim234 Senior Member

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    That was my thought too. Because it came so closely after the conditioning, though, I really feel more comfortable confirming things with people here. I have a lot of respect for the Priuschat members who really know the ins and outs of these cars.
     
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  13. uart

    uart Senior Member

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    Yes, that's exactly the problem with manually calculating the MPG over a single tank. The fuel quantity that you use in your calculation is NOT the fuel that you actually used over the period for which you are calculating the MPG, it is the fuel that you WILL use (or at least have available to use) over the NEXT period!

    This is the reason why an anomalously low calculated MPG is quite often followed by an anomalously high MPG on the very next fill, and vice versa.
     
  14. Sam Spade

    Sam Spade Senior Member

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    You can NOT count on the automatic nozzle to shut off the same every time.
    If the gas splashes a little bit differently, it can shut off earlier.
    Damage causing deformation at the end of the spout can make it act differently.

    Don't panic. See what happens next.
     
  15. 05PreeUs

    05PreeUs Senior Member

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    That's my guess, the HV battery temperature plays a BIG role in it's ability to accept and provide power. Once you started driving with the AC on, it took a LONG time to get that much mass under 100*F.