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MPG Full Tank vs Half Tank or Less

Discussion in 'Gen 4 Prius Fuel Economy' started by JimmyPri, May 5, 2016.

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  1. JimmyPri

    JimmyPri Active Member

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    This might be common knowledge among Prius owners so forgive me for asking. I seem to be getting almost 10 mpg better with near a half tank of gas than when I had a full tank. Will it get even better as I go towards empty? Since gas weighs 6+ per gallon thats 30 pounds. Is that enough to get 10mpg better? This is my first tank of gas. Could it be a combination of things such as me also learning to drive the Prius way? I have noticed that I also have been looking for alternate routes back up the hill to where I live - such as streets that do not have as steep a climb even if it is only a slight difference.

    I used to fill up on my ICE cars when the tank got just below half. It looks like I should wait until it is closer to empty. I seem to remember ages ago when there was the thought that if a gas tank got near empty you can possibly pull up impurities from the bottom of the tank. Is that relavent these days?

    If one wanted to get every mpg you could, why fill a tank to full unless you had a long trip to go on? Would a constantly half empty tank cause sweating and therefore moisture in the tank (and more trips to the gas station). Or are the tanks designed not to do that? This is just a question not a statement. I am looking for various opinions.
     
  2. bisco

    bisco cookie crumbler

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    no question, physics, isn't it. i stay between 2 gallons and empty. that means from the flashing pip to zero. and i only add two gallons cause i need to drive 120 miles straight sometimes. also, i remove everything possible from the car that is unnecessary weight. and, i'm on a diet.:cool:

    one of the best things about the gen 4 is that they eliminated all the storage ares that were gas hogs.
     
  3. JimmyPri

    JimmyPri Active Member

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    I don't weigh too much myself, so I can't go on a diet. But I have been practicing jumping or popping up in the seat so that half the time I am in the air therefore my weight is only a drawback half the time. But the seat belt holds me back???
     
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  4. bisco

    bisco cookie crumbler

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    you wear a seatbelt? i pulled all mine out to reduce weight.
     
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  5. max451

    max451 Junior Member

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    The Eco model dropped ~65 lbs (plus aero improvements) and only got 4mpg
     
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  6. CR94

    CR94 Senior Member

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    30 pounds is less than 1% of the mass of the car plus yourself, so carrying 30 pounds less fuel will reduce fuel consumption while accelerating or on an uphill almost proportionately, i.e., by less than 1%. Any other time, like level cruising, the difference would be a lot less than that 1%.

    Yes, you should wait until the tank is closer to empty to refill it, not because that would save significant fuel, but to save yourself the inconvenience of needlessly frequent fuel stops---assuming of course you don't go so far to the opposite extreme that you risk running out of fuel.
     
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  7. wjtracy

    wjtracy Senior Member

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    My wife would be down to 35 MPG if she had me in there.
     
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  8. alanclarkeau

    alanclarkeau Senior Member

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    I'd be surprise if the extra mass made much difference. I fill each time, that gives the best point of reference for fuel efficiency.

    On a new car, the impurities from the bottom of the tank should be minimal risk.

    I'd start with a full tank (I go to the first bowser click, trying to fill at the same bowser each time - not always possible) to enable you to accurately measure fuel use. Fuel gauges and hence computer readouts could be less or more inaccurate at different parts of their scale.
     
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  9. KrPtNk

    KrPtNk Active Member

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    To the best of my knowledge, a fuel injected engine maintains a continuous fuel pressure by having the pump run constantly. Fuel circulates constantly from the tank to the injectors and back again. Any fuel not burned is carried back to the tank through a return line. This constant flow would keep the fuel in the tank well mixed and filtered. I wouldn't worry about contents at the bottom of the tank being any different than the top. Besides, the fuel supply comes from near the bottom of the tank anyway.
     
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  10. pakitt

    pakitt Senior Member

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    I would test this over several tanks and average. Assuming ambient temperature remains the same or about the same in the trips you take between tanks and between full and half full tanks.
    I don't think that 20L/5gal will make a huge difference in fuel consumption so much...hence, try multiple times over several tanks, note the ambient temperature and the routes you do, and see if you can notice such a big difference after all.
     
  11. wjtracy

    wjtracy Senior Member

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    Previously I have posted GM's rule-of-thumb approximation equation..

    I forget the number but let's say less than 1 MPG loss per 100-lbs. So for a 200-lbs me maybe 2 MPG loss. FYI I do not weigh 200-lbs I rounded up.

    So the reason I am NOT re-posting GM's equation is that it makes no sense compared Toyota saying Li batt saves 40-lbs and 4 MPG. Something is not adding up.
     
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