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Something's not adding up?

Discussion in 'Gen 2 Prius Fuel Economy' started by icameasdust, Apr 4, 2014.

  1. icameasdust

    icameasdust New Member

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    Hey guys, first post here on priuschat, so bare with me!

    But anyways I recently purchased my first 2007 Prius. I've been getting about 400 miles a tank, which I'm not complaining about whatsoever. Well, a few days ago, I filled my tank all the way like usual. Well, here's my problem; something isn't adding up at all. I'm down to exactly half a tank now, trip is only at 99 miles, and my average on this trip so far is 49mpg. But if you do the math, I'm actually getting 20mpg according to the reading. Could something be wrong here?
     
    #1 icameasdust, Apr 4, 2014
  2. Easy Rider

    Easy Rider Active Member

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    The something that is wrong is that you are trusting gauges that really are only "estimates" to give you accurate readings. That is a mistake. They won't.

    You really have NO WAY to tell that you are down to "exactly half a tank".
    The gas gauges in the older models are well known for being erratic sometimes.

    The ONLY way to gauge your MPG is to divide miles travelled by gallons used over several fills.
    Sometimes even that will be off a bit as gas nozzles don't always shut off at the same point.

    The numbers displayed by the computer shouldn't be trusted on an absolute basis but the relative numbers are usually pretty good. That is, if the computer says your usage has changed, then it probably HAS; you just can't be sure exactly how much.
     
    #2 Easy Rider, Apr 4, 2014
    Epiphany2000 and icameasdust like this.
  3. usnavystgc

    usnavystgc Die Hard DIYer and Ebike enthusiast.

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    The gas guage on the Gen II is notoriously inaccurate. The tank is actually a bladder that shrinks and swells in capacity. Additionally, the pump may have shut off on you earlier than normal this time around. I personally found the mfd mpg calculations pretty close to correct when I was driving my Gen II every day. Just to be safe, you should refill when the guage nears empty.
     
    #3 usnavystgc, Apr 4, 2014
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  4. dorunron

    dorunron Senior Member

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    What usnavy answered is what I would say. As stated the fuel gage is not the best in the world. Fill up when it gets down to two or three pips. And the bladder will vary in volume due to temperature and other factors.

    Over time you will get used to the quirks of the system.

    It will take a few tanks of fuel before you see what we are talking about. The MPG at the pump versus the MPG on the MFD will vary also due to the bladder. Again, that is normal. You will also find that the MFD reads a little higher (usually about 2 MPG difference) than your true "overall" MPG average when measured at the pump.

    Sounds like your Prius is fine. Don't worry about it.

    Get a fuelly.com account and keep track of your mileage there. It is free and will help you to get a better idea of your true and actual mileage.

    If you want some more info on your Gen II, click on the links below in my signature file. I believe the information will help you.

    Best of luck to you,
    Ron (dorunron)
     
    #4 dorunron, Apr 4, 2014
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