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Troubleshooting: Fuel gauge vs mpg

Discussion in 'Prius c Fuel Economy' started by CSuirp, Oct 6, 2018.

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  1. CSuirp

    CSuirp Active Member

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    Hey all. Question. Running a 2014 Prius C trim 3 with around 52k miles. With hopes of staying on top of mpg, I reset the trip B meter when I saw a square bar disappear on my fuel gauge. The car went 38 miles till the next bar disappeared. I did the test again and same results, 37, 38 miles till the next bar vanished. I took a look at the monthly average mpg graph on one of the settings and it says I'm doing about 55mpg (average) per month, and the daily driving average on dashboard shows I'm doing about 55mpg daily. Trip A meter reads about 270 miles on 1/2 tank. So, at this point I'm not sure what to believe. Am I actually only getting 38mpg (based off what the trip B meter shows me after each fuel bar vanishes) or am I getting 55mpg (based off the monthly drive average graph and the daily drive mpg screen on dash)?? I thought 1 fuel bar represents 1 gallon of gas, but not sure. I spoke to Toyota dealer and they want to inspect the fuel tank and diaphragm claiming that maybe it's not operating correctly due to temp changes, but we haven't had drastic changes in temp in the last 6 months. If the car is actually getting 38mpg...does anyone know what should be looked into to fix this? I read frozen calibers can cause major drag...perhaps that?

    Please help shed some light on this for me if you can. Thanks and sorry if I'm overlooking an obvious issue or I'm in error with my fuel gauge assumptions.

    PS...background of vehicle:
    *4 new fuel saver tires installed about 4 months ago, standard 15" rims
    *Tires inflated with correct pressure
    *Driving a mix of highway and city with little use of A/C and rolled up windows for most of driving.
    *No short trips being done.
    *No additional body kits added that may conflict with natural aero design.
    *Conscious driver who maintains level in EV and ECO range on city driving
    *Slow brake distance stops, steady accelerations, seldom into the PWR range
    *Highway is speed limits around 55-60.
    *Cruise control often used on highways doing around 58-60.
    *No weird sounds on car or noticeable engine issues. Just slight phase vibration in the 60's mph range.
    *Toyota did inspection on last oil change (3 weeks ago) and everything passed.
    *Changed cabin filter (pretty sure i put it with arrows pointing correctly)
    *Changed engine air filter
    *Cleaned battery intake screen in back seat area
    *Battery gauge remains at 1/2 to 3/4 full most of the time.
    *No heavy items being lugged around daily. Car is relatively empty 24/7.
    *Gas is 89 octane or better
     
    #1 CSuirp, Oct 6, 2018
  2. fuzzy1

    fuzzy1 Senior Member

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    NO! NO! NO! You are trying to read far too much into the meaning, accuracy, and linearity of the gauge. Don't do this, it is not a precision measure.

    For 'customer expectation management' reasons (i.e. to reduce complaints to the customer support folks), the gauge doesn't even show the full range of the tank. The bottom of the gauge is higher than the actual bottom of the tank, to provide some safety margin and reduce ID10T errors from drivers running out of gas. The top of the gauge is usually a bit lower than the actual top of the tank, to give rental customers a bit of slack in filling up on the way to the rental lot.

    And the bar display is filtered so that the sloshing fuel level in the tank (from hills, corners, stop and go, etc.) doesn't show up as multiple oscillating bars on the display, but this filtering produces some inconsistent bar transition levels.
    Also an improper way to compute actual gas mileage.

    Until your first fill-up, just use the daily display or a trip meter (when reset at fill-up time) MPG display.

    Keep a fuel log. At your first fill-up, compute MPG as (miles driven since previous fill-up) / (gallons added). This will be your real MPG, and is commonly slightly lower than the 'optimistic' trip display.

    For this issue, ignore them, they will only lighten your wallet without really fixing anything. Do the math above after a refill, and report that number back to us.

    If you really do have a brake drag problem (I've had it in two previous cars, one from frozen caliper, one from shop snafu), it will also show up as an overly hot wheel hub or brake disk. Or in a really bad case, smoke, followed by flames.
     
    #2 fuzzy1, Oct 6, 2018
    Last edited: Oct 6, 2018
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  3. dubit

    dubit Senior Member

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    wow.....

    That display your looking at is an estimation, not an EXACT indication of your mpg's. Which in a nutshell is what Fuzzy said up there. Just write your mileage down on your gas receipt when you fill up next. If you want to keep track of this, click the link to fuelly.com in my post and set yourself up an account. I kinda get the feeling you'd be a good fit for the place. :)
     
    #3 dubit, Oct 6, 2018
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  4. sam spade 2

    sam spade 2 Senior Member

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    This. Is all you really need to know.
     
    #4 sam spade 2, Oct 6, 2018
  5. Mendel Leisk

    Mendel Leisk Senior Member

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    Or really archaic, at each fillup: note the miles driven since last fillup, the gallons of gas pumped, divide the miles by the gallons, that is your miles per gallon.
     
    #5 Mendel Leisk, Oct 6, 2018
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  6. CSuirp

    CSuirp Active Member

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    Thank you Fuz for taking the time to type all that out. I'm glad you guys are here to help out other Prius owners. I'll cancel that dreadful appointment with Toy now and crunch some mpg #'s and see the results before going any further. Much appreciated!
     
    #6 CSuirp, Oct 6, 2018
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  7. CSuirp

    CSuirp Active Member

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    Thanks Dubit!! I'll check out the fuelly.com!!
     
    #7 CSuirp, Oct 6, 2018
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  8. CSuirp

    CSuirp Active Member

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    Thanks Mendel!! Appreciate the calculating advice!
     
    #8 CSuirp, Oct 6, 2018
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