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Gas Gauge bars vs. computed MPG

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by christob, Mar 20, 2023.

  1. christob

    christob Member

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    Hi folks. First off, apologies for another post on the gas-gauge pips situation; I did spend some time reading through old PC forum posts before this, but couldn't readily find an answer.

    2007 Prius purchased new, now 97,000 miles.
    My mileage (as displayed on the center console) seems to be pretty constant over the last 15 years. I reset the console stats on every fill-up. I see 49-53 MPG (console calculated MPG) in summer driving and I see 43-47 MPG in winter driving.
    By that reckoning, I've grown accustomed to "a summer tank is good for 375-400 miles" and, "a winter tank is good for 325-350" before I start looking to fill up... (Generally thinking ~8 gallons between fill-ups.) In fact, this often corresponded to fill-ups in the 7-8 gallons range. A 9 gallon was pretty rare for me, and I don't think I ever hit a 10 gallon fill-up. I generally don't wait for blinking 1-pip on the gas gauge; I will usually fill up at 2 bars. Now (~last 2 years?) the gas gauge runs down to 1 bar much more quickly, usually before 280 miles travelled. Well before the computed-MPG against tank-miles-driven would suggest I should be running low -- assuming an 11.9 gallon tank.

    Example from this week:
    Filled up fresh for a 415-mile-long interstate road trip. (oops - I put more gas in, topping up after the pump-shut-off... Something I've done on probably 50% of my fill-ups over the last 15 years... sigh; missed the memo not to do this practice!)
    After 298 miles driven, the gas gauge started blinking 1 pip. I recall reading that the blinking is designed to start when approx. 2-3 gallons remain? So blinking would imply I used up approx. 9 gallons; which would yield here, a trip-MPG of just 33. But the computed MPG was showing 47.1 for the trip. Based on that computed trip MPG I should have only used ~6.3 gallons; a far cry from nearly emptying the tank to trigger the blinking. But out of caution I stopped for a fill-up; and took in ~6.5 gallons til the pump clicked off. So that would seem to support the console calculated MPG is pretty accurate for determining how many gallons were used for the miles driven...

    My question - is there a way to tell if my tank is in fact still able to hold ~11.9 gallons at a fill-up? And if it is still holding a proper, full tank, then in this trip's example, it seems I could have travelled say, 423 total miles (47.1 x 9 gallons) before looking to fill up?
    IF that were true -- I gather the blinking pip would simply have continued blinking at me for another 125 miles of travel...?
    I guess I'm not sure -- is the blinking 1 pip a legit indicator of true remaining gas available -- implying my tank no longer holds close to the original 11.9 gallons, and the systems are correctly telling me I will in fact soon run out of gas and the car will stop...
    Or, has the gas gauge fallen out of synch / no longer reliably indicates the amount of gas remaining -- but the computed MPG figure still serves as a reliable way to determine gas consumed so far (and thus forecast the remaining travel miles available) based conservatively on aiming for 9 gallons burned between fill-ups?

    I mean, I really want a 2023 Prius Prime, but I'm not ready to buy a new one just yet ;) Thanks in advance for your input...!
     
    #1 christob, Mar 20, 2023
  2. mr_guy_mann

    mr_guy_mann Senior Member

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    Either possibility could be valid. Maybe the bladder is stiff enough that useable capacity is reduced, OR the gauge sending unit is worn some and is even less accurate than it used to be.

    The only way "to know for sure" would be to bring a full 5 gallon gas can and then keep driving the car until it stopped. Most aren't THAT interested in finding the answer since there can complications from; having the ICE stop suddenly, getting safely out of traffic, not draining the HV battery, getting the fuel system bled out, etc.

    Me, I fill up when I get a flashing pip, 6+ gallons in winter, 8+ gallons in summer.

    Since any repair involves replacing the fuel tank assembly, I don't care about until I have to.

    Posted via the PriusChat mobile app.
     
    #2 mr_guy_mann, Mar 20, 2023
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  3. sam spade 2

    sam spade 2 Senior Member

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    Yet another case of OCD over an inaccurate gauge in a car. o_O
    Fill up more often.
    Get over it.
    Seriously.
     
    #3 sam spade 2, Mar 20, 2023
  4. christob

    christob Member

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    Such a helpful reply... thanks for taking the time!
     
    #4 christob, Mar 20, 2023
  5. fuzzy1

    fuzzy1 Senior Member

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    The Gen2 fuel tank bladder adds a huge amount of uncertainty. Short of finding a time/place/circumstance to safely and deliberately run the tank all the way to fuel starvation, it is unlikely that you'll get a satisfactory answer. Among previous Gen2 owners who have run their tanks very low, their experiences have been spread across a wide gamut, from numerous consistently getting significant distances after the last bar starts blinking, to some others running out with a still-solid bar, a few with even 2 bars. It seems that each fuel bladder shrinks or shrivels or folds (or not) differently.

    That is his normal style. That is why there is an 'Ignore' function.
     
    #5 fuzzy1, Mar 20, 2023
    christob likes this.
  6. sam spade 2

    sam spade 2 Senior Member

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    What I said is the honest truth.
    Some people can't handle the truth.
    Have a nice day.
     
    #6 sam spade 2, Mar 20, 2023
  7. christob

    christob Member

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    Thanks, done and done!
     
    #7 christob, Mar 20, 2023
  8. Tombukt2

    Tombukt2 Senior Member

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    I've run my Gen 2 out of gas like twice on purpose to see if there would be any issues and both times that this is been done I've been right at my property car ran out of gas red triangle came on All of that I knew immediately what it was obviously. Turn the car off by the button got out put in 2 gallons of measured fuel of high test because that's all that's here out of the proper fuel container stored in the shade under a shed etc after doing that this was immediately after running out of gas after putting the gas in sat in the car push the button didn't even wait for the car to start or anything put it in reverse to back up like I'm going to get ready to go somewhere The car knows this and we back up by the time I put it in drive the engine started without any funny business and after reading from others here of not such an easy time I tried it again a few months later same place and everything 2 gallons of gas backed up car started right up no issue ice running. My Corolla is due this the same way there's no priming no doing anything
     
    #8 Tombukt2, Mar 20, 2023
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