inaccurate sure, inconsistant, why?

Discussion in 'Gen 3 Prius Fuel Economy' started by billnchristy, Apr 18, 2012.

  1. billnchristy

    billnchristy Active Member

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    My estimated MPG is so horribly out of whack that it is not even funny anymore. I have gotten used to it being a couple MPG off, no biggie but this last sequence has got me to the point of annoyed.

    Last tank MFD displayed 46.6 and calculated 44.8mpg.

    Typical, no biggie, probably last of the winter gas.

    Filled up yesterday, MFD displayed 48.8 and calculated 44.9mpg

    :confused:

    Its annoying when you see 50+ and just go..."yeah, right"

    BTW 2010 with 56k miles
     
  2. F8L

    F8L Protecting Habitat & AG Lands

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    Are you topping off such that you can see fuel in the filler neck? If not then chalk it up to inaccuracies with the pump clicking off. :)
     
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  3. spiderman

    spiderman wretched

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    I have gotten use to the inaccuracy and just take into account when I look at it. Sad, but that is just the way it is.
     
  4. qbee42

    qbee42 My other car is a boat

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    Have you tried averaging over a large number of tanks? I'm not convinced that the inconsistency is the meter; it's more likely to be the manual filling technique.

    Tom
     
  5. billnchristy

    billnchristy Active Member

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    Never go past first click on pump.

    Molly (Toyota Prius) | Fuelly

    I'm not questioning my actual MPG, I think it is about right. I am questioning why one week the MFD can read 46 and the next 49 and the actual be no different.
     
  6. Codyroo

    Codyroo Senior Member

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    I'd wait for my next fill up or two before I'd pass judgement on the HID displayed value. It could be that the current fill up put in an extra 0.1 to 0.2 gallons into your tank. It may be that the next fill up, your displayed value is 48.8 and your calculated value is between 46 - 48 mpg. Did you have any tanks prior to this where you had an unusually close difference between your displayed and calculated MPG values?

    I've tracked my MPG's for my Prius for 2 years now, and can think of a couple of instances (not many) where the tank got extra gas on one fill up and then got a normal fill up on the next. Despite the determinate error in the displayed value, I can calculate my gas needs at the pump within 0.1 gallons (I'm not half bad at math in my head). I know pretty quickly if it shut off early. I managed to stop a flow the other day just as it started to overflow the tank because I realized I was already 0.15 gallons past my expected stop point. Just as I was letting go of the handle, gas was bubbling out (damn it). I've been very wary of that pump since.
     
  7. billnchristy

    billnchristy Active Member

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    Never been within 2 but I have grown to expect that.

    I didn't go much past 0 so I was expecting 9 or so. Maybe the pump went .2 over which is enough to really hose things up (would've been 46 then and close to what I would expect given the usual difference between gage and actual.)
     
  8. Felt

    Felt Senior Member

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    I generally buy my gas at the same station, and try to use the same filling technique. Still, there is some unexplained variation.

    But I have also experienced a very convincing situation where the pump was way off. With the price of gas being what it is, I suspect that there is the potential to "adjust" the settings (like dialing the pay-out on a slot machine). Yes, the state checks ....but realistically, how often can the state check every pump? I just have my doubts.
     
  9. fuzzy1

    fuzzy1 Senior Member

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    I'm still not seeing what the real problem is. In my use, variations in weather and driving conditions easily throw the actual mpg around more than that, so the MFD display will follow.

    And tank filling variations easily throw the difference between MFD and single tank calculations around by that much. None of the parking pads at my usual stations are level, each pad is tilted different, and since installing a ScanGauge on the previous car I've seen variations that seem to follow the vehicle tilt while refueling.

    Is your car level during refueling?
     
  10. ETC(SS)

    ETC(SS) The OTHER One Percenter.....

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    I always hand-calculate tan, k by tank. When I fill up I go to the second click. It's not 100-percent accurate either since as pointed out earlier your clicks may vary. If you're THAT worried about it, then always drive until your DTE allows you to pump exactly 9.something gallons of fuel, and always pump THAT amount. Then, you only have to worry about being consistent about when you get fuel. Example: DTE=0, get gas within 10 miles. Over time, your "tank" ranges should become fairly accurate
    Or.....you could try to drive exactly X miles and then pump X number of gallons. Then, your only variables are odometer inaccuracies, which are slight, and pump delivery inaccuracies, which should be even more minuscule.
    You're not giving up much in tank range, and it will allow for more accurate guestimations without having to worry about trashing the Evap system or over filling your tank.

    My last three or four tanks, I've noted that I always pump 9.4-something gallons, since I always drive until I get the warning chime and then hit the nearest gas station. It's easier for me, since I don't pay for the fuel and I don't have to scratch around for the cheapest station.

    Anyway...you get the idea.
    The MDF????
    I don't even look at the MPG numbers anymore. They're nearly valueless....pun unintended.

    I'm not sure what level of accuracies you're trying to achieve, My last 3-4 tanks have been well inside a tenth, and I'm covering three counties.

    Anyway.....good luck.
    Ignore the MDF.
     
  11. billnchristy

    billnchristy Active Member

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    It's not a big deal it's just kind of silly that a 24k car that is a technological marvel can be so horribly off.

    My 16k Fiesta is always within 1mpg but usually within .5 and I have never experienced a wild variation like I have with the Prius.

    Like I said, not a big deal, actual MPG is what it is. It sounds like they got it figured out with the C anyways so at least they are getting better.
     
  12. JimN

    JimN Let the games begin!

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    Different pumps have different sensitivities. I found one in Pittsburgh PA that kept shutting off. I found one in Delran NJ that didn't shut off. I found one in greater Mt. Holly NJ that shut off late resulting in crappy FE for the rally. (That one hurt.)

    The ONE TIME I pay cash I pumped in 20c worth to get to a dollar after it clicked off. I learned that a 60mpg tank cost 20c.

    A little bit of gas either way wrecks havoc with the numbers but it all averages out in the end.
     
  13. mad-dog-one

    mad-dog-one Prius Enthusiast

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    It seems to me that your numbers show that the MFD is reliably invalid, consistently showing slightly poorer MPG than what you calculate by dividing miles driven by gallons of fuel replaced. Some of the difference in reliability that you attribute to the MFD measured mileage is due to invalidity of your measure of replaced gas.
     
  14. billnchristy

    billnchristy Active Member

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    You're right. Next time I will wear my lab coat and pump all the gas into graduated cylinders.

    That should fix it.
     
  15. szgabor

    szgabor Active Member

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    This is GenIII specific not prius in general my GenII was within 1 mpg !!! almost always !!! I had it two years or so and it was correct not just by tank but any 5 tank combined ... did not even bothered to have error calculated

    GenIII is 3-7% errors over the 75 fill-ups error is about 4.5% plus ALWAYS more (GenII was up and down a little so overall was very accurate)

    Toyota engineers must be better than this I smell intentional positive bias built in to make you feel happier with the 24k car :)
     
  16. Codyroo

    Codyroo Senior Member

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    Actually, you should use Volumetric flasks, they are more accurate, just make sure when you check the meniscus that you have your safety glasses on. ;-)
     
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