Is the fuel gauge to be trusted?

Discussion in 'Gen 4 Prius Fuel Economy' started by Greg M, May 25, 2018.

  1. Greg M

    Greg M New Member

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    Hello all,

    I've had my 2016 Prius Four for 8 weeks now and am loving it so far. Bought it used with 21K miles on it. Thanks to all who have posted so much helpful information on this forum.

    Here's my question: First tank of gas got me 575 miles before I had to refuel (with still about a gallon or so in the tank, I'm guessing, based on what the manual says about the low fuel warning). I'm nearly 3 weeks in to my 2nd tank now, and the numbers seem off--I'm just not sure which one(s) are accurate and which aren't. According to the display, I've traveled 315.6 miles, averaging 68.3 mpg, and still have over 5/8 of a tank remaining. No matter how I do the math, this doesn't add up. These numbers can't all be right. I know the mpg numbers are typically inflated 2-3 mpg, but that actually makes less sense.

    Let's say I'm getting 65mpg. That means I've used 4.8 gallons. So shouldn't I be down to almost half a tank of fuel left? Is the gauge bad? Or am I missing something here.
     
  2. fuzzy1

    fuzzy1 Senior Member

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    Maybe you are overthinking this, or reading too much into the gauge?

    Most automotive fuel gauges are not meant to be precisely mapped to miles driven or miles & fuel remaining, especially without (or before) personally measuring and calibrating your particular car to your wants. Which you haven't yet driven far enough and refueled enough times to have done.

    Traditionally, the top of the gauge is not at the real top of the tank. The bottom of the gauge is not at the real bottom of the tank. And in between, the bars are not perfectly linear. Both my Prius and my newer Forester have astoundingly good fuel gauges compared to all my 20th Century cars. But still not good enough to have the math you are trying to do, accurately add up.

    I believe you will just need to keep track of your gauge marks vs refuel amounts and trip odometer distances, and personally 'calibrate' your own fuel gauge. And after filling up, don't forget to watch how long it takes to drop below 'Full' or below the very top bar. On many older cars, this is surprisingly long, and is built in as part of 'customer expectation management'. It helps allow car renters to fill up off-airport and still have the gauge show Full when they turn it in.
     
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  3. Mendel Leisk

    Mendel Leisk Senior Member

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    Just use the gas gauge as a rough guide to how much is left in the tank.

    To determine your mpg, fill up, the same way each time, divide the amount of gallons indicated on the receipt into the distance travelled since the last fill up.

    Any inaccuracy in a single fill up calculation will virtually balance out by the next, and with 3 or 4 tanks the accuracy will be deadly.
     
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  4. alanclarkeau

    alanclarkeau Senior Member

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    This has been discussed on PriusChat - yes, they're not accurate - but then, most car petrol gauges aren't.

    TOYOTA advises to only fill to the first pump click. What particularly messes your figures up is if the pump prematurely clicks off, with a few litres below where it normally clicks off. The gauge will probably still appear to be full, but will drop off quicker. It will also reset the km-to-empty figure, but because it wasn't really full, doesn't give an accurate reading.
     
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  5. sclevine

    sclevine Member

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    In my experience, it takes about 75-100 miles before you see the gauge tick down below full, so you have to imagine the gauge going even further off to the right in the full direction - ie: pretend it is 9ths or 10ths rather than 8ths. It can be confusing, but based on rentals I've also driven, most new cars are like this. My old 2011 VW was the same exact way.

    Once you see the gauge start to drop below full, it goes down pretty linearly down from there. Do you have a 'miles to go' counter? I find that to be pretty accurate and much easier to follow than basing it on the percentage left in the fuel gauge.
     
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  6. padroo

    padroo Senior Member

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    I have a 2016 Prius and had a 2008 before and I noticed the new one moves in smaller increments.
     
  7. Kramah313

    Kramah313 Active Member

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    There’s also the matter, at least in the gen 3, of the car still having the 2 gallon reserve when the gauge shows empty. So there are 10 pips on the gen 3, and the car has almost 12 gallons. It kind of works out to 1 pip per gallon.

    I know you have a gen 4, and I think instead of pips it’s a continuous bar, but I bet the “philosophy” Toyota used is similar.
     
  8. Elektroingenieur

    Elektroingenieur Senior Member

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    The New Car Features book says the fuel tank capacity is 43.0 L, and the Repair Manual has a chart, under Vehicle Interior: Meter/Gauge/Display: Meter/Gauge System: Fuel Receiver Gauge Display Malfunction:

    Fuel receiver gauge indicates F: 38.3 L
    Fuel receiver gauge indicates 3/4: 29.8 L
    Fuel receiver gauge indicates 1/2: 21.3 L
    Fuel receiver gauge indicates 1/4: 12.8 L
    Fuel receiver gauge indicates E: 4.3 L​

    The manual also says, “The fuel sender gauge assembly cannot detect changes in the fuel level within certain ranges (around points E and F),” and it explains how the system responds to level changes from refueling, driving on hills, and the like.

    (The Repair Manual and New Car Features books are available by subscription to techinfo.toyota.com.)
     
  9. Mendel Leisk

    Mendel Leisk Senior Member

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    Seems like the halfway point is fairly accurate, and the two extremes are "compressed" a bit.

    Whenever anyone bemoans the "innacuracies" of the gas gauge, I wonder, just what are they hoping to glean/achieve from that gauge. I suspect it's mostly to faciliate running the gas tank perilously close to dry, which is pointless I think.
     
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  10. RRxing

    RRxing Senior Member

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    Another question would be - Did you fill up that first tank of gas or did it indicate "FULL" when you bought it? The tank may not have been "FULL" if you didn't fill it up yourself, hence the slightly lower value. As mentioned, don't rely 100% on the car's gauge, make paper calculations (I add the info to my spreadsheet - more data = more accurate) each time you get gas for a better representation.
     
  11. Greg M

    Greg M New Member

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    Thanks--that's helpful.
     
  12. Greg M

    Greg M New Member

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    I'm not "bemoaning" anything. I'm asking a question, looking for advice. I thought that was one of the purposes of this forum. And your suspicions are incorrect--just trying to learn more about a car that is new to me.
     
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  13. Andyprius1

    Andyprius1 Senior Member

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    Can also be disastrous, never run out of fuel. At the very least, charge up the EV portion, just in case.
     
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  14. Mendel Leisk

    Mendel Leisk Senior Member

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    Point taken. With a few tanks, you'll get a feel for how the tank compares to the displays. Personally, I refill any time it gets below half and my station choice is convenient. But that's admittedly at least 3 weeks between fillups, very low usage.
     
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  15. bisco

    bisco cookie crumbler

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    how do you 'charge up the ev portion'?(n)
     
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  16. alanclarkeau

    alanclarkeau Senior Member

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    Most old gauges were very inaccurate - would often fill well past full:

    upload_2018-5-27_12-32-17.png upload_2018-5-27_12-36-52.png

    AND well below "E" when empty:
    upload_2018-5-27_12-38-28.png

    I'm not sure that PRIUS is any more accurate. To indicate on a gauge less than 50mm wide, the contents of a 40+ litre fuel tank will necessitate it being relatively approximate.
     
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  17. Andyprius1

    Andyprius1 Senior Member

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    The HV Battery might be considered the EV portion Vs the ICE, the non- EV portion.
    One never charges the ICE.
     
  18. alanclarkeau

    alanclarkeau Senior Member

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    For those without a PRIME (which the OP hasn't), you can't force the charge on the HV Battery - unless you climb a large hill and use Regen all the way down, but in normal driving, it'll end up sometimes with 6 bars, other times 2 bars.
     
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  19. first_superior_prius

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    You're missing the fact that the fuel bars are in 1/8th increments and the difference between 4/8ths (1/2) and 5/8ths is one bar. It's probably just barely over half full, which both your math and the fuel gauge are reflecting. With such discrete chunks of measurement, it's impossible to be hyper accurate. It's "close enough" and you shouldn't overthink it

    If you want a (slightly) more accurate estimate, use the distance to empty reading for a more real-time guesstimate
     
  20. RCO

    RCO Senior Member

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    What I did on arriving at hotel parking before a 2 week cruise with HV showing just 2 bars SOC at 20°C was turn the heater up to max and open the car's windows. With car Ready and in P, the engine fired up and charged the HV battery to 2 below full when I was satisfied all was in order and reset to normal before shutting down. Two weeks later when we docked again, car had not lost much charge at all. Hotel staff said there had been a UK heatwave while we were away so I was best pleased with my solution. :cool:
     
    #20 RCO, May 30, 2018
    Last edited: May 30, 2018
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