2015 WRX AVG MPG gauge problem
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This is a discussion on 2015 WRX AVG MPG gauge problem within the New Member Hangout forums, part of the Community - Meet other Enthusiasts category; I have 2015 WRX base model and I just found out the the instant avg mpg gauge(the middle one) fluctuates ...

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    2015 WRX AVG MPG gauge problem

    I have 2015 WRX base model and I just found out the the instant avg mpg gauge(the middle one) fluctuates down by less than 1 and keeps increasing....it was reading around avg 60mpg and keeps climbing up with slight fluctuation. I initialized the gauge to factory condition and still doing it. I'm getting 40mpg avg on city lol and if I drive longer, it will keep increasing. there is no check engine light or anything. I made an appointment to check that out with Subaru dealer. Is this normal?
    Last edited by 6spdtls; 04-18-2015 at 12:07 PM.

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    something else forever Germz to Japz's Avatar
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    haha it would be funny if you actually were getting that high mpg but whatever... kinda impossible lol...

    doesn't sound normal... hopefully the dealer can figure it out... good luck!
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    Quote Originally Posted by Germz to Japz View Post
    haha it would be funny if you actually were getting that high mpg but whatever... kinda impossible lol...

    doesn't sound normal... hopefully the dealer can figure it out... good luck!
    It started happening after driving down the hill from mountain. After spending a couple of more hours or so driving freeway and city, instant avg mpg was reading lot more accurately. numbers were still little high though which read 40freeway and 30city. maybe it's recalibrating itself?

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    something else forever Germz to Japz's Avatar
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    Quote Originally Posted by 6spdtls View Post
    It started happening after driving down the hill from mountain. After spending a couple of more hours or so driving freeway and city, instant avg mpg was reading lot more accurately. numbers were still little high though which read 40freeway and 30city. maybe it's recalibrating itself?
    that could be... i mean you just got the car right? is it still in the break-in phase? sometimes things take some time... not sure though... this sounds kinda strange...
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    Quote Originally Posted by Germz to Japz View Post

    that could be... i mean you just got the car right? is it still in the break-in phase? sometimes things take some time... not sure though... this sounds kinda strange...
    yeah i got the car a couple of weeks ago. 800miles on it at the moment. I asked a couple of other subaru dealer mechanics and describe the symptom and they both told me car seems pretty normal to them.

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    something else forever Germz to Japz's Avatar
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    Quote Originally Posted by 6spdtls View Post
    yeah i got the car a couple of weeks ago. 800miles on it at the moment. I asked a couple of other subaru dealer mechanics and describe the symptom and they both told me car seems pretty normal to them.
    if they say it's normal then it's probably the system and electronics just getting used to something... no idea... just keep driving it smooth and safe until 1k, get that oil change, and then you're good to go...

    hopefully after that it's all settled and fixed up
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    Instant average mpg issues are probably either a software bug or a sensor malfunction. As long as your fuel gauge is reading accurately, it's just an annoyance.

    The running average mpg is always going to be off... Rather than simply tracking how much distance you cover and how much fuel you consume and diving miles by gallons, it probably adds up each instant reading and divides by the total number of readings. Even if the instant readings are correct, this method gives you the wrong answer. Example: Suppose you have a 20 mile trip up a steep climb to your favorite ski area. On the way up you get 10mpg and on the way down you get 40mpg. If you average those two mpg numbers you get 25mpg. But, if you add up the total amount of fuel used... 2 gallons on the way up and half a gallon on the way down... you realize that you used 2.5 gallons of gas to travel 40 miles. Your average mpg for the round trip is 40/2.5= 16mpg. This is an extreme example, but it demonstrates how higher samples skew the gauge's version of the average. (Look up geometric mean if you are curious as to why simple (A+B)/2 doesn't apply)

    I find that my '05 Outback's MPG meter "lies high" by about 2mpg unless I'm doing a long trip on flat roads in which case it lies high by about 1mpg.

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    Instant AVG mpg gauge started acting normal again. I have no idea what caused it but I was really pissed off. The Subaru dealer mechanic told me there isn't any sensor in any car that measures instant avg mpg and it's based on consumption of fuel economy.... so my claim of "faulty sensor" was totally wrong lol??? WTF! So I asked him "does 99.9 mpg seem reasonable to you?" and he said "yes, it sometimes fluctuates....and seem normal especially if you are driving downhill". I was speechless lmao
    Last edited by 6spdtls; 04-19-2015 at 09:09 AM.

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    Quote Originally Posted by 6spdtls View Post
    So I asked him "does 99.9 mpg seem reasonable to you?" and he said "yes, it sometimes fluctuates....and seem normal especially if you are driving downhill". I was speechless lmao
    If your foot is completely off the gas pedal and you're moving at highway speeds, that reading is likely accurate. "Instant" mpg means "right now", it's an average sampled over a very short time (whichever way they calculate it). If your foot is off the gas and you're moving at highway speeds you're consuming a piddly amount of fuel while moving at a good speed, so 100mpg during that time is not only believable but expected. If you're looking at the running average and seeing 99.9 it's almost surely faulty. (Unless of course you reset your trip odometer just as you start coasting down a long hill...)

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