Fuel Gauge Issue?

Discussion in 'Gen II Prius Fuel Economy' started by reese2147, Mar 27, 2012.

  1. reese2147

    reese2147 Junior Member

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    Hello

    I wanted to ask the forum if some observations with my fuel gauge seem to be "normal".

    I am a new to a Prius, purchasing a 2007 less than a week ago. I topped off on fuel and have been driving my commute normally. The gas gauge seems to act erratically in terms of how it "loses" fuel. This morning, my prius hit 1/2 tank (according to the gauge) at 175 miles. This would mean I would only be getting 350 miles to the tank of fuel (or roughly 31 MPG). However, the electronic MPG indicator shows me at 47 MPG on average for the tank of fuel. During the fuel gauge's trip to 1/2 full, it seems to drop a "bar" on a sporadic basis.

    I found this post on the forums describing what seems to be a similar issue.

    "issue with fuel gauge???" - Posted November 2011

    I wanted to ask if what I am describing also seems to fall within the category of "normal" for the "guess gauge". If so, are there any tips for handling this issue?

    Thank you for your help!
     
  2. ursle

    ursle Gas miser

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    :welcome:
    Your gas tank has a plastic bladder that the fuel tends to expand during warm weather and the bladder tends to not expand so well when the temp drops, as you're in a northern climate and the weather has cooled off your bladder is feeding you false info about it's capacity. If your mpg average isn't low then wait for warm weather and it will act differently.
     
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  3. reese2147

    reese2147 Junior Member

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    Thanks for the reply!

    I was thinking it may have something to do with temperature. Last week, it was in the high 60s/Low 70s temp wise. Today it was 30 degrees during my commute. Would probably explain the drop in my MPG during my commute as well? (51.5 MPG when warm out compared to 47 MPG today in cold weather?)
     
  4. lamebums

    lamebums Member

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    Hi reese2147--

    Yep, the temperature can make all the difference. A colder start means a longer warm-up period (and the Prius, even more so than other cars, will practically dump fuel into the engine until it reaches at least 104 degrees). Plus, especially if you drive on the highway, cold air is denser and provides more wind resistance - sometimes as much as 10%.

    I've witnessed the same thing, as it was in the 80's all last week here, and now it's barely cracking 40 - and my tank average is dropping slowly after each trip I make, despite my attempts to drive efficiently.
     
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  5. stevemcelroy

    stevemcelroy Active Member

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    As others have said the gauge is not terribly accurate and the bladder means that there is no set size for the tank. In the fall, winter & spring I figure that my tank can take about 9 gallons. In summer it goes up to the full 11+ gallons. I have found that even during warm spells like last week the tank does not loosen up - the full capacity is really only feasible when the weather is hot.
     
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  6. reese2147

    reese2147 Junior Member

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    Thank you for all the feedback.

    I have one last question on this issue.

    Since he fuel gauge is not very accurate and the amount of fuel the vehicle takes can also vary, what is the best way to gauge what my fuel mileage is? I've read that the on-board MPG indicator has some level of variance to manually calculated MPG.

    For some background, I have a 100 mile round trip commute each day for work. I'm not looking for 100% accuracy, but is the on-board MPG indicator a fairly accurate representation of the mileage I am getting? I continue to be slightly perplexed by this tricky fuel meter and am concerned with never really knowing how much fuel I have or when I need to fill up.
     
  7. JimboPalmer

    JimboPalmer Nadir of Wrongness

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    With temps in the 80s, I am getting 10.5 gallon fill ups again.

    The MFD is pretty accurate in the Gen 2, get gas once you hit 2 pips, and certainly by 1 pip.

    If you track by multiple fill ups, the miles / gallons calculations are accurate, just not for any one tank
     
  8. qbee42

    qbee42 My other car is a boat

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    The MFD is pretty accurate for the Gen II Prius. Use it for your day to day information. If you want to hand calculate, log your miles and fills and calculate using multiple tanks. At five tanks the error gets manageable; at ten tanks it's effectively gone.

    Tom
     
  9. lamebums

    lamebums Member

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    Hi reese2147--

    I'm probably one of the few who isn't (currently) singing the praises of the MFD because my last tank was hand calc'd at 46.7 when the MFD said 62.x. The majority position seems to be that the MFD is 2-5% optimistic, meaning if it claims you're getting 50, you're actually getting 48-ish.

    You can also get a ScanGauge, which can also show instantaneous MPG (although I haven't corrected its' calibration from my prior vehicle yet).
     
  10. qbee42

    qbee42 My other car is a boat

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    A single hand calculation is worthless for the Gen II Prius.

    Tom
     
  11. reese2147

    reese2147 Junior Member

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    Thanks again for all of the great feedback.

    While out at lunch today, I noticed that my fuel gauge read 2 bars. I proceeded to the gas station. From the two bars, it took 6.4 gallons to "fill up". I had traveled 320 miles since my last "fill up" for a manual calc MPG of 48.4 (which ironically, was slightly higher than the on-board MPG read of 47.6 MPG).

    I suppose it truly is defined best as a "guess gauge" huh? ha ha

    I'll continue a few manual calculations as suggested, but this initial one has be optimistic that the vehicle is getting decent MPG for how I drive. This leaves my only concern with.... never really knowing how "far" I can go.
     
  12. JimN

    JimN Let the games begin!

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    Welcome to the "Guess Gauge". I used 1 pip to represent 1 gallon. The only time you should believe a fuel gauge is when it is pointing at Empty.

    A good rule of thumb: Consider buying gas at 2 pips. Buy gas at 1 pip. When the last pip starts flashing BUY GAS NOW.

    Probably, almost all the time you'll still have another gallon or more of usable fuel. Maybe one time you won't & find yourself stuck on the side of the road in the middle of nowhere.

    Enjoy the highs & don't get too bummed on the lows. A high tank followed by a low tank indicates a fueling error. You really have 2 average tanks.

    I expect the Scangauge gets the same data as the MFD so I wouldn't expect improved accuracy. In fact, you have to enter the tank size which can't be done accurately.

    Divide the miles driven by the tank's FE as reported by the MFD. Have you burned through these many pips (1 pip=1 gallon)? It should be close. As I remember I figured the MFD was 5% high.

    On a fixed commute it doesn't matter. You know where the gas stations are. Cross country it's an issue and can lead to some pretty anxious miles.

    Invest in a ScanGauge. With this week's cold snap it's not too late to block the grill. You'll want the gauge to monitor inverter temp. Might as well increase the tire pressure. These will give your numbers a boost.

    If you are going to keep track you really need to build a spreadsheet to record every tank. Expect wide variations. My file is attached for reference.
     

    Attached Files:

  13. dorunron

    dorunron Senior Member

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    Hi Reese,

    As other's have said, it is a waste of time to try to get a "decent number" for the MPG on one tank.

    On the '09 Second Gen Prius we have, the MFD will display anything from low 40's to low 50's. Right now it is reading just over 51. A lot of variables effect the Prius fuel economy: wind, temperature, humidity, tire pressure as well as the condition of the vehicle.

    I have found that the best way to accurately calculate the true MPG of the Prius is to keep track of several tanks over a long period of time. After doing this for four months, the Prius that we own has an average of 46.625 MPG. I do expect that number to be higher as I have recently redone the front brakes as the inboard pads were stuck on the glide pins which caused additional friction which naturally would lower the mileage.

    To make a long story short, the MFD seems to be pretty close if average over several tanks. The number will vary from time to time due to many things including your driving habits.

    Also just because one person gets 60 and you only get 45 does not mean you are doing something wrong. What it means is that the person getting 60 is driving under different conditions with different habits than you.

    Unless you are getting really crappy mileage 30 or below, I would say just enjoy the ride and remember what you see on your gas pump readings in comparison to the guy next to you with that big SUV gas hog. At today's prices, I am seeing folks spending $80.00 and more at the pump. I bought gas yesterday at $3.889 per gallon, the guess gage had four pips on it, and the pump stopped at $16.05. I always :) when I fill the Prius. It takes twice that much to fill my Nissan P/U or more @ half tank.
     

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  14. AZGeek

    AZGeek Semi-informed Member

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    Give me a break. :mad: How many gallons does it take to fill up once the last pip starts flashing? If it takes 11 or more then I would agree with your plea to BUY GAS NOW. But, and I am willing to bet body parts on this one, very few people will be putting in even 10 gallons at that point. Assuming poor economy at only 40 mpg, you still would have over 50 miles to travel before you needed to freak out. Most folks will struggle to put in 9 gallons as soon as the pip starts to flash.

    Please, the last pip flashing doesn't mean you have to stop what you are doing to buy gas. :rolleyes: Relax. Get to know your car and fill-up patterns.
     
  15. cwerdna

    cwerdna Senior Member

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    Each car varies. Some people have run out very quickly after the last pips starts flashing. To quote from http://priuschat.com/forums/gen-ii-prius-fuel-economy/102210-2-pips-got-380-miles-2.html
    To quote from http://priuschat.com/forums/gen-ii-prius-main-forum/33751-have-you-run-out-gas-in-your-prius-4.html
    From http://priuschat.com/forums/gen-ii-...ran-out-gas-after-472-miles-5.html#post196691
    From http://priuschat.com/forums/gen-ii-prius-fuel-economy/24085-low-fuel-indicator.html#post301692
    I've never run my Prius out of gas. I see no reason to chance it when down to a flashing pip. The most gas I've ever put in at a time was 9.975 gallons. That was my 1st tank. I don't recall how low I ran it (1 pip or flashing pip).

    Maybe http://priuschat.com/forums/other-cars/94001-gas-gauge-says-full-but-thats-not-quite-true-npr.html will give you some more perspective, even though it's a Ford engineer talking.
     
  16. enerjazz

    enerjazz Energy+Jazz=EnerJazz

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    Here is 7.5 years of data showing the calculated MPG, the carPC MPG, and the bias. Due to the bladder variability the car PC is more consistently accurate if you figure out your car's bias and apply that.

    See the bias chart on this page: Prius Data
     
  17. AZGeek

    AZGeek Semi-informed Member

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    I understand that each car varies, hence "Know thy vehicle". However, to go on total miles on the tank is not wise, because your economy on that tank is what really matters. Folks in cold climes getting mid-30's / low 40's mpg shouldn't expect to be getting 400+ miles on a tank.

    So all of the posts that only talk about miles on the tank don't mean much. What I am trying to say is that you don't have to get gas right away with the pip flashing as long as you know what your car's been doing with the mpgs. I always figure the MFD is off by 10% and estimate how much fuel is still on board based on the mpg (adjusted) and the miles driven.

    The one exception to this is when I have had the car in for service and don't know how the car has been kept (always on, ICE kicking in, etc.). Then I get pretty conservative.

    One other factor to keep in mind is understanding how your current driving conditions compare to those that were present to establish the current mpg calculation on the MFD. If you averaged 55 for the first 440 miles (8 gallons) and then drive like a bat out of hell, you aren't going to get 55 mpg for the remaining 3 or so gallons. Again, "know thy vehicle".

    I am just tired of folks claiming you HAVE to get gas once the pip starts flashing as if were a blanket statement for all Prii. If you want to get gas as soon as the pip starts flashing then do so. :)
     
  18. AZGeek

    AZGeek Semi-informed Member

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    :rockon::rockon::rockon: