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MPG low when coasting?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by Heather Nae, Oct 10, 2015.

  1. Heather Nae

    Heather Nae New Member

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    Okay so I wasn't sure if I should put this in the Fuel Economy forum or here. Hopefully I chose right.

    So I have had my 2010 Prius since the very end of August. I drove it to Minnesota from Texas and back. When I got to Minnesota I found out the dealer had completely lied about topping off all my fluids as my coolant was SUPER low and already needed an oil change after having the car only one week.

    Well about two weeks ago I was leaving work when I noticed that the bar that shows immediate mpg (What do you call that thing?) was showing only 3 pips when I wasn't touching the gas pedal at all. The problem varies around that. Sometimes it's around the '50 mpg' mark for the first few minutes after I start the car, sometimes it is much lower. Overall my average mpg hasn't taken a big hit because after a few minutes, the car goes mostly back to normal and it will indicate '100 mpg' when coasting/breaking.

    It does seem that when looking at the Hybrid System Indicator screen, when I go over the gas line, I'm not positive, but I'm pretty darn sure that the indicated mpg is not as high as it used to be (before this sudden problem) and drops immediately when it goes over the middle line. I know it's going to drop when I start using gas but I know it should not be quite as low as it is. (Sometimes around 25 mpg when it's just a quarter of the way over the middle line.)

    I haven't been driving this car long but I have driven it a lot so I feel pretty used to what I know the screen should be indicating to me. I know there's some warm up time needed when first starting the car but this has to be something else as it seemed like a sudden and immediate change. I drove home from work, everything seemed normal. I drove to work, everything seemed normal. I drove home from work and suddenly there was something wrong.

    I've read about an auxiliary battery and I kinda feel like that might be the culprit but I really don't know enough about this to say anything for sure. And if it is the battery, is there a chance the dealer might have known something was wrong with it and decided to just give it a bit of a charge until some poor chump took the car off their hands and they wouldn't have to worry about it? That would seem a bit silly seeing as they're only about, what? $150? A lot for me but doesn't seem like much for a dealership. Although I guess every dime counts for their books.

    Any tips, questions, advice will be very appreciated! I've been stressing about this problem, hoping it would magically go away or was a fluke but after persisting for over two weeks, I realize that was pure naivety on my part.
     
  2. David Beale

    David Beale Senior Member

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    Perhaps what you are seeing is the low MPG you get at low speed. If you are "coasting" with the engine running at lower speeds, such as in a parking lot, you will see poor mileage indicated. The idling engine is using fuel, but you are not covering much distance per time, hence poor mileage.

    I doubt the 12V battery would be causing this.
     
  3. bisco

    bisco cookie crumbler

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    welcome, great avatar!:) i would start tracking mpg's by the tankful and report here. i think your just being a little anxious about your purchase and the dealer. read some mpg threads to get a feel for what others are getting, and start learning the in's and out's of the car like oil level, tyre pressure, 12 volt battery and etc.
    no need to worry about instantaneous mpg's. all the best!(y)
     
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  4. Ozark Man

    Ozark Man Member

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  5. bisco

    bisco cookie crumbler

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    exactly what i was thinking!
     
  6. Ozark Man

    Ozark Man Member

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    I bought my Prius as Certified which meant I received a check list that all of the fluids had been checked. I don't know how I missed it but when I arrived home I noticed that the brake fluid was low. I was concerned but when I filled it, it never went down again. As far as that indicator of MPG is concerned I would just go by the real mileage that you get and try not to worry about it. That thing is all over the place. I know it's hard not to worry though, when you pay good money for a vehicle you expect everything to be perfect like they promised.
     
  7. FuelMiser

    FuelMiser Senior Member

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    Instant MPG is useless. A better way to monitor average MPG between fill-ups is to select Trip A (or Trip B) with the TRIP button on the steering wheel, then once selected, hold the TRIP button to reset the counters at the bottom of the HSI to zero. This will then maintain the running miles, average MPG and average MPH until you next zero it with the TRIP button. Also, when you shutdown the car, the display will show your stats from the current trip (distance covered, average MPG, average speed, fuel cost based on the per gallon cost you enter, and total miles on the car).
     
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  8. The Electric Me

    The Electric Me Go Speed Go!

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    Prius gauges offer so much feedback in regards to immediate gas mileage that I often think new owners can be mislead.
    I wouldn't worry about how many pips are showing when you have your foot off the accelerator.
    Sometimes if the engine is cold, it will run, to warm itself up. This will happen in idle if you are just sitting still.

    I always tell new Prius Owners to pay attention to full tank MPG averages. And do it over the course of a month or longer. If you start worrying about all immediate feedback? You're likely to imagine problems where there are none.

    If after 4-6 weeks you have HORRIFIC or substandard MPG? Then you might have a problem.
    But tankful averages...and more than one...is the way to go.

    I think of all the immediate feedback as Prius Software encouragement to drive frugally. It can be helpful, but I don't even worry anymore about what The Prius say's my MPG for a short trip or what the MPG immediately happens to be. It's just going to vary a whole lot, and can be influenced drastically by variables as out of my control as outside temperature.
     
  9. JimN

    JimN Let the games begin!

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    Heather, welcome to the club. Never trust the dealer. Before accepting delivery I check the fluids. Easy enough for the prep person to check it off the list then not pour something in the car. My Intrigue was delivered without windshield washer fluid. I presume by now you replenished whatever fluids were low with the correct fluids.

    A weak 12v battery isn't going to cause a significant drop in fuel economy. You really have to be paying attention and looking for the additional charging. I didn't notice the additional charging on the old battery but I did see less charging on the new one.

    If you have the NAV or AT package you can run a battery diagnostic among many other things.

    As the others have mentioned, keep track of your fuel economy tank to tank. Cross country driving is not a good model for local commuting. I believe you just need more than 5 weeks to get used to the car and for the car to get used to you.
     
  10. bisco

    bisco cookie crumbler

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    nice talking to you heather.
     
  11. CR94

    CR94 Senior Member

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    Low instantaneous mpg with your foot off the accelerator is perfectly normal and ok when the car is moving slowly while the engine is running to warm up. It's also normal any time you're accelerating, especially at low speeds. After all, instantaneous mpg is simply the car speed divided by the fuel burn rate, which can't be zero with the engine running.

    What are you getting for average fuel consumption, according to the "CONS." display, or by manual calculation?

    Good luck, and don't worry about it too much! You can make up for the low mpg numbers with high numbers at other times.
     
  12. dorunron

    dorunron Senior Member

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    Not sure what part of Texas you are in, but I will say this. With the seasons changing the symptoms you are describing sounds perfectly NORMAL to me.

    What you really should do is what everyone else is saying here. Go by the big picture (MPG for the whole tank of fuel). The instantaneous read out is going to vary from time to time. It is what it is. In my opinion, you are getting more accustomed to the vehicle and beginning to notice the subtle differences in the modes of operation. Again, it is all normal.

    Now if you find that your MPG over a full tank or two is less than say 30 MPG, then you do have a problem. Anything from 40 and up is pretty much normal for the Prius.

    I am posting a pdf file which you should read. It discusses MPG in greater detail and applies to the Prius.

    Best of luck to you and "Welcome to Prius Chat"! (y)
     

    Attached Files:

  13. Heather Nae

    Heather Nae New Member

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    Thank you guys so much for replying and the welcomes! You're all too kind!

    I've been trying to tell myself I was just being overly anxious and am just not used to what to expect yet. I know the car has a warm up period and since I have a short city drive to work I will not be getting the super high gas mileage like I dreamed, but I was expecting that.

    I just ran a 12 v batter test and it is slightly lower than it should be but nothing significant.

    And it is usually warm out which I know effects the cars ability to perform however I left for work today before the sun was up and was seeing the same 'problem' that I have been at 3 o'clock with the sun beating down. I do have an app where I've been loading fill ups and mileage so we'll see what I'm actually getting. as for now my gas gauge is saying that I have half a tank and I would guess I've driven between 100 and 150 miles so no that's not great, but we'll see what my average is after a month or two and I will report back!

    I really hope you are all right and this is perfectly normal and it's all in my head! I will try to remember what you've all told me so I don't stress as much about this!