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Thread: Trip Computer MPG

  1. #1
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    Trip Computer MPG

    How many people calculate their true MPG by hand and compare it to the trip computer?

    My trip computer ALWAYS grossly overestimates the MPG I'm getting. It's usually not even close to the actual MPG.

    I drove 412 miles on my last tank...
    - Trip computer said 35.0 MPG.
    - Actual gas mileage was 32.5 MPG. It overstated the fuel economy by 2.5 MPG!

    That has been the most egregious overstatement, though it is almost always off by 1.5 - 2.0 MPG.

    My guess is the Mazda engineers (correctly) assume most owners do not check their mileage by hand.
    So they can program the trip computer to state wildly optimistic MPG numbers, and consumers will be none the wiser.

  2. #2
    Registered Member 7eregrine's Avatar

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    This gets asked about every 3 months.
    I tracked mine for 3 months and it was always within 1.

  3. #3
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    I've been tracking fuel economy since I got my car about a year ago, and I'm with bluegrass on this one. My manual calculation is always lower than stated by the on board computer. Not a big concern for me, since it is within my personal acceptable range.

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    Mine isn't a CX-5 but a CX-3 though I think the computer is pretty much the same.
    My actual mpg is usually (about 95% of the time) within 0.5 MPG US, rest is of by 0.5-1.5.
    Sometimes the actual mpg is better than the computer, sometimes worse.
    Mind you I've had to convert from L/100km to MPG US, but that shouldn't change anything.

    So no, I don't think Mazda has programmed their computers to be wildy optimistic.
    Mazda CX-3 Optimum 2.0 Skyactiv-G 120HP Automatic

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    Registered Member Conrad 16.5's Avatar

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    I always track my fuel economy in all my vehicles. I have a lil notebook in each one and it all gets entered, along with my maintenance notes.

    That said, I never double check the stated economy to the actual economy. What's the point? It's just for reference. What would I do if the two numbers didn't agree? That's right, nothing.
    "Don't bother me with facts, Son. I've already made up my mind." -Foghorn Leghorn

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    Quote Originally Posted by Conrad 16.5 View Post
    I always track my fuel economy in all my vehicles. I have a lil notebook in each one and it all gets entered, along with my maintenance notes.

    That said, I never double check the stated economy to the actual economy. What's the point? It's just for reference. What would I do if the two numbers didn't agree? That's right, nothing.
    If the trip computer was actually accurate, you wouldn't need to calculate it manually every time.

    And it would provide a realistic MPG reading for the 98% of people who don't calculate manually.

  7. #7
    Registered Member 7eregrine's Avatar

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    A lot of people calculate manually because it is so ridiculously easy nowadays.
    This app:
    https://play.google.com/store/apps/d...m.kajda.fuelio

    has been installed over million times. And there are many others.
    And here's one for the Apple heads:
    https://apps.apple.com/us/app/fuelly...er/id295905460

    I always track my fuel economy in all my vehicles. I have a lil notebook in each one and it all gets entered, along with my maintenance notes.
    1995 called bro and wants you to get an app. Some of them synch to your PC or let you download it all if you want. Notebooks?!?
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  8. #8
    Registered Member Kaps's Avatar
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    Quote Originally Posted by bluegrass View Post
    If the trip computer was actually accurate, you wouldn't need to calculate it manually every time.

    And it would provide a realistic MPG reading for the 98% of people who don't calculate manually.
    The car calculates the mpg based on certain measurable factors but it never calculates based on actuals. It never calculates the volume of fuel used and uses that it approximates it based on some sort of a formula.
    what this does - some people get the same mpg as their car gets. Some get less and some get more. When I was keeping note - my real world was .5 to 1.25 mpg more than what my car showed - and this was due to my commute cycle and my drive style. I tend to coast a lot. It depends on your driving style / commute and other factors.

    This inaccuracy isn't so much of an issue for me - it seems that different CX-5s have different thresholds for the fuel light. I have never filled more than 11.5 - 11.8 gallons and I think that is just too conservative. I would love my CX5 to let me drive and use up 12.5 gallons at the very least. Atleast drive 350 miles before needing a fuel station.

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  9. #9
    Registered Member yrwei52's Avatar
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    Arrow Trip Computer MPG

    Quote Originally Posted by Conrad 16.5 View Post
    I always track my fuel economy in all my vehicles. I have a lil notebook in each one and it all gets entered, along with my maintenance notes.

    That said, I never double check the stated economy to the actual economy. What's the point? It's just for reference. What would I do if the two numbers didn't agree? That's right, nothing.
    Like you I have a little notebook and have been written down the same information in it.

    Iíve checked the computer calculated MPG only few times so far, and the difference is within 1 MPG most of time but computer calculated MPG is always higher.

  10. #10
    Gen-1 Kodo Design 👌 ColoradoDriver's Avatar
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    I've never tracked my MPG on this car in the 6 1/2 years I've owned it. I look at the dash tracker every now and then and all I know is that it's a lot better than the barely 15 MPG I was lucky to get on my last car!
    2014 CX-5 Touring AWD 2.5L | Jet Black Mica - 86k miles
    01/2013 build | 4/20/2013 delivery

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    Quote Originally Posted by Kaps View Post
    what this does - some people get the same mpg as their car gets. Some get less and some get more. When I was keeping note - my real world was .5 to 1.25 mpg more than what my car showed - and this was due to my commute cycle and my drive style. I tend to coast a lot. It depends on your driving style / commute and other factors.
    You're the first person I've heard from whose trip computer underestimates MPG.

    There was an article in Car & Driver that showed discrepancies between trip computer vs. actual MPG, across many brands/types of cars.
    In every case, the trip computer always indicated better MPG than the vehicles actually got.

    Regardless of my commute or driving style (in town driving, aggressive rush hour commute, steady highway cruising, ect), my trip computer ALWAYS reports considerably higher MPG than actual.

  12. #12
    Structural Member shadonoz's Avatar
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    Quote Originally Posted by Kaps View Post
    The car calculates the mpg based on certain measurable factors but it never calculates based on actuals. It never calculates the volume of fuel used and uses that it approximates it based on some sort of a formula.
    How do you know that? Can you tell us more about this method or where to learn more about it? I'm trying to imagine how it would work without measuring the fuel.

  13. #13
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    Quote Originally Posted by bluegrass View Post
    You're the first person I've heard from whose trip computer underestimates MPG.
    Well then let me be the second, because I consistently get better mileage than what the IP shows.
    ---------------------------------------------------------------------
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    Quote Originally Posted by hw4ng3r View Post
    I've been tracking fuel economy since I got my car about a year ago, and I'm with bluegrass on this one. My manual calculation is always lower than stated by the on board computer. Not a big concern for me, since it is within my personal acceptable range.
    Me too, usually about 1 MPG, although I stopped checking most of the time months ago and just accept the mean shift.

    I usually fill 13+ gallons, I start thinking about getting gas when the light comes on unless I have a long trip planned.

  15. #15
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    I've been keeping mine in a spreadsheet every time I fill up so I can track mileage by octane & brand. I have about 4,400 miles on my car and am on my 17th tank of gas.

    Any variances between the computer and manual calculations even out the next tank or two of gas.

    It only takes a little under-filling the tank when you start off to make it look like your mileage declined (because it took that much more gas to fill up at the end of that tank). Conversely, if you under-fill some at the end of the tank, it looks like your mileage is higher than the computer stated because it seems you went through a lower amount of fuel.

    It's not hard to do. Not all gas stations are perfectly level.

    But (surprisingly) the computer is accurate.

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