calculated vs. computer MPG - Please post your results

Discussion in 'Gen 3 Prius Fuel Economy' started by F8L, Jul 10, 2009.

  1. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Do you normally run higher tire pressures? Dealership might have dropped them to spec, which would make a slight difference.

    Was any brake work involved? Brake screw-ups, in particular with rear brakes, can leave the car with dragging brakes. After a longish drive, feel all the wheels and see if any are markedly hotter. Typically they would only be very slightly warmer than ambient temp, all around.

    Any software updates indicated?

    37 mpg calculated is really low, especially for a car with 15" rims. Anything else that might account for it, say excessive idling, lots of short trips, someone else (with a leadfoot) driving it?
     
  2. jerrymildred

    jerrymildred Senior Member

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    Welcome.

    The list of possibilities is long. @Mendel Leisk already mentioned some of them.

    Also, a failing 12V battery will put a pretty good load on the charging system and make more impact than one might expect.
    Engine and/or cabin filters can hurt you.
    A dirty MAF sensor.

    One of the biggest drains on fuel efficiency is hard stops. Hard starts waste fuel, but hard stops just flat throw it away. But in many cases, traffic demands hard stops.
     
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  3. Michelg2

    Michelg2 New Member

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    Hello Mendel Leisk, Thank you very much for your reply. As far as for the tires goed, these are new and following spec pressure. Don't think this should be the case.

    You mentioned something very true and that I have not realized, in the last 3 months due to the pandemic outbreak I have been driving lots of short trips, I would say 130 miles in April 90% short trips and another 200 miles in May of another 90% short trips.

    I will to my brake flush change today together with fuel induction service. I will let you know the difference in the coming days.

    Thanks.

    Hi Jerry,

    Thanks for sharing your thoughts. 12V battery makes me definitely think about it. I don't even know when it was last changed and the fact that the car has hardly be driven in the last 2-3 months with only 1 or 2 rides per week has possibly it has lost performance.

    Let's see how it continues after this new full tank.
     
    #1283 Michelg2, Jun 5, 2020
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  4. Salamander_King

    Salamander_King Senior Member

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    I am having bit of trouble figuring out what you did to calculate your mpg here. Is your reading of 360.8miles from tripmeter reset at last fill-up or odometer difference? Or did you get the number from the DTE range? Either the tripmeter or the odometer readings are accurate miles driven but DTE range is not. Still, your on dash mpg is considerably higher than your calculate mpg which is ~11% higher.

    Not exactly sure what it means by "fuel induction service", but I can almost guarantee it is not needed and will not fix the poor gas mileage problem.

    Short trips do hart the mileage a lot especially in cold climate. Since you are in Florida, cold may not be a problem. But if very hot and using A/C on every short trips, it is conceivable you get very poor mpg. Did you ever have better mileage than say 42mpg since you bought it used? You mentioned "New" tires. Did you put the new tires? New tires always shave few miles off from mpg. Also types of tires affect a lot. If it is a new non LRR tires, that could be one factor. Without knowing the service records of the former owner, it is difficult to guess what service have or have not been done.

    As one date point, this is a graph (blue lines) of my Gen3 (2015 Prius Two) hand calculated mpg when I had it for the first 35k miles from new. As you can see new Gen3 is capable of getting above 50mpg but it can have bad days of down to 40mpg range mostly during winter in NE. My life time average on this car over 35k miles was 49.2mpg.

    upload_2020-6-5_11-11-38.png
     
  5. ASRDogman

    ASRDogman Senior Member

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    Fill the tank. Reset one of the trip meters to zero.
    Drive until the tank is have empty/full. Fill the tank, use the same station and hose if you can.
    Take the miles traveled and divide by the amount of fuel added. That will be your MPG.
    250 miles divided by 5.5 gallons equals 45.4545455 miles per gallon.
    THIS is only an EXAMPLE!
     
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  6. Michelg2

    Michelg2 New Member

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    Thank you. This is exactly how I did it but with empty an empty tank. I will try next time with half deposit since I have reset the tripmeter.
     
  7. Michelg2

    Michelg2 New Member

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    The 360.8 miles is the reasing from my tripmeter after puting it to zero on my last fill up. It is not the odometer difference or DTE range since I know this last is not accurate. And yes, very much higher, that's the reason of my concern.

    I think and I hope main cause of this low performance are the amount of short trips I have done in the last 3 months. I did have better mileage above 42 mpg when I bought the car but never touched the 50 MPG. I would say they were between 47 and 45 MPG. Reading with about 50 MPG I only get when the summary trip information is shown after turning off the engine but this is not the average. My new tires are a ZEETEX, not the best I think and they are not claimed to be Low resistance. Another reason perhaps.
     
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  8. Mendel Leisk

    Mendel Leisk Sand Pounder

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    One more tip: just for insurance, record the odometer reading when you're filling up. It can never be accidentally reset. Also, if someone "helps you out, put's in $10's worth", as long as you can get the receipt, with the amount added, you can roll with the punches, do the calc when you next do a full fill up.

    Whenever you turn off the car, the odo displays, for about 20~30 seconds.
     
    #1288 Mendel Leisk, Jun 5, 2020
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  9. ASRDogman

    ASRDogman Senior Member

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    It doesn't matter how much fuel you have when you refill. But usually, you'll get a more accurate
    reading when you go farther. I just used that as an example.
    Basically, you reset the trip meter when you fill the tank. Then drive X amount of miles, then refill
    and divide the miles traveled by the gallons fueled.

     
  10. Michelg2

    Michelg2 New Member

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    Hi Guys, what do you think about this ?

    Do you think I have to change my 12V battery and that this is definitely the cause of my low MPGs ?

    Any feedback and thoughts are highly appreciated!
     

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  11. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Sorry for the tone, anyway:

    Off-topic.
    The car displayed voltage is pretty much meaningless; test with an electronic load tester.
    That said, looks like it's toasted.
     
  12. Michelg2

    Michelg2 New Member

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    So a 12 V bat change would be recommended ?
     
  13. jerrymildred

    jerrymildred Senior Member

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    As @Mendel Leisk said, testing to see if it needs to be replaced would be recommended. If it's always reading low, it's probably shot, especially if it's the original. FL is brutal for batteries. There must be a thousand discussions here about testing batteries. If the forum's crummy search feature isn't working (I rarely find what I need with it.), search using Google and include "priuschat" in the search terms. Much better results that way.
     
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  14. sadie72

    sadie72 Junior Member

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    My posted miles to empty have been running at 410+ since moving to Chicago. In the ATL I was at 510+ on every fill up. Now after I disconnected the battery to replace all brake pads and rotors, the Prius is showing 600 miles until it hits reserve which is approximately 2 gallons. I don't know. Maybe it was that fuel injector cleaner that I added at last fill up... LOL.
     
  15. Michelg2

    Michelg2 New Member

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    Hi @sadie72 I wonder what MPG is showing your dashboard? very possitive. I would like to test that injector cleaner too though! WHich product was it exactly and how did you add it? Thanks!
     
  16. dancingbalu

    dancingbalu Junior Member

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    I always have 0,3 Ltr / 100 km more when i calculate it then it was displayed from the trip computer.
     
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  17. black_jmyntrn

    black_jmyntrn Member

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    From my findings, cleaning the EGR helps improve MPG.
     
  18. abuwaseem

    abuwaseem New Member

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    i have 35mpg i change hybrid battery 1 year ago i clean egr and i checked the catalyst its good but why my mpg is low i use some pulse and glide but its same

    Posted via the PriusChat mobile app.
     
  19. rhs64

    rhs64 Junior Member

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    FCD always 3-5% higher, marketing will be served. 2012 Prius 79k
     
  20. pakitt

    pakitt Senior Member

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    My Gen3 was 6.5% off (optimistic). My Gen4 was 4.6% off. I am guessing the Prime is still around 3-5% optimistic.
    My father's Honda Jazz and Insight were always off by 1-2% (the same with the odometer.)

    There is also the fact that what you put in the tank is not necessarily what the gas stations tells you they sold to yo...and all sorts of other variations/errors due to temperature change, calculation rounding errors, etc.

    But the above data is based, respectively, on 63k and 21k miles worth of fuel ups, at different gas stations, in 3 different countries, so that should average all the above errors out.
     
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