Miles to Empty not accurate?

Discussion in 'Gen 3 Prius Main Forum' started by Silvertn350, Oct 13, 2015.

  1. 348

    348 Junior Member

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    It's just an estimate as the car has no idea if you will be driving up a mountain or down a valley in the next minute.

    Your Prius only knows the terrain under the wheels and makes estimates in the present, it has no idea what is coming in the next mile. The same with your MPG display. The only way to get your real MPG is math when you fill up.

    ........bummer for recent victims of our Public Education System
     
    #21 348, Oct 13, 2015
    Last edited: Oct 13, 2015
  2. Codyroo

    Codyroo Senior Member

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    The DTE (distance to Empty) is a very rough guide and isn't very accurate at all (other than to ensure that you fill up your tank prior to running out of gasoline).

    If you want a "little" better estimate, you can take your tank MPG and multiply it by 10 and you'll have a nice estimated tank range. If you are in for a REAL THRILL, multiply it by 11 and when you hit that value, you will be on fumes (or empty)! This assumes that you have a 5% error on your displayed MPG, like my car.

    For example, if your displayed MPG is showing 50.0 mpg, you can expect to go 500 miles and then need to fill up. It is highly likely that you will put in 10.4 - 10.6 gallons when you fill up, leaving a buffer of ~1.5 gallons or so. If you go 550 miles, you'll less than 0.5 gallons and may run out of gas.

    I've tracked my gas mileage for 120,000 miles on my 2010, and my displayed MPG is 5.7% higher than the actual MPG. This system is easy to do and can be easily calculated at any point in your drive (displayed MPG x 10). It does take some of the panic out of a flashing PIP and DTE = 0 miles.
     
  3. Redpoint5

    Redpoint5 Senior Member

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    I'm seeing about a 4% optimistic MPG reading. It will show 52 MPG when I'm actually getting 50.
     
  4. tpenny67

    tpenny67 Active Member

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    Going back to the original question, yes it is common for the miles to empty number to drop dramatically shortly after filling up. Here's what I think is actually happening: MTE is actually recent MPG * gallons remaining in tank. However, when you first fill up the computer has to assume how many gallons are in the tank, because the gas gauge isn't terribly good at distinguishing between 95% full and 100% full. Basically, when the tank is really full the float hits the top of the tank and can't register anything less than 100% until the fuel level drops a bit.

    After you drive a number of miles and the fuel level does drop, the computer can get a more accurate estimate of the fuel in the tank and will update the MTE accordingly, most likely in a gradual manner over a number of miles. So depending on whether you get the tank somewhat full or really very full, the initial MTE may be somewhat off.

    At the other end, once the float hits the bottom of the tank, the gauge can't measure the last little bit of gas sloshing around in the tank, so all cars have have always had a bit of range after E. Modern cars seem to have added some significant extra buffer into this, so E now means "start thinking about filling up".

    BTW, this doesn't apply just to the Prius, my Escape does the exact same thing. Ford even mentions in the owners manual that miles to empty is based on the MPG over the previous 500 miles.
     
  5. qdllc

    qdllc Senior Member

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    Even in a regular ICE vehicle, any "estimate to empty" is based on guess work. If you were to floor the gas, the number would drop while you're doing that, and as soon as you throttle back to 35 mph, it'd jump back up.

    Think of it like a GPS estimation of your "arrival time." It's based on what the GPS knows about the programmed route and where you are in relation to the destination. If you're going 20 mph over the limit, the arrival time will always be sooner than displayed until you're a few miles out. At that point, the estimation factor relies less on pre-set values and more on actual data.
     
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  6. Oskar

    Oskar Member

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    Yeah, have to admit I did this until I was consistently getting between 55 & 60 mpg. No big deal now. I know exactly what my car is going to produce in terms of efficiency and economy, so I now just drive it instead of worrying how many mpg I'm going to get.
     
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  7. David Beale

    David Beale Senior Member

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    There is no fuel filter. There is a "fuel strainer", non-replaceable wire mesh. Any "gunk" in the tank will either stay there or be pumped through an injector. The level you drain the tank to has little to no effect on whether your fuel pump will pump whatever is in there into the fuel system. The intake tube is always at the same level. In the "gunk" or out of it.

    Yes, the fuel pump uses the fuel as a coolant. I have never heard of any Prius owner who ran out of fuel having to replace the fuel pump due to any reason. There is a thread on this forum about what happens to a GIII Prius if you run out of fuel. Bottom line, avoid it!

    So I agree, running the tank low is not very good practice. Pearl S gets filled up when she gets to half a tank most of the time. I don't care how much the fuel costs, because I buy so little the price difference is just a few dollars max. for the fillup. I will leave it longer on a highway trip, but not much below 1/4 left.

    You won't have problems with "condensation" in the fuel tank. It's a "sealed" system. The only water in there is water you pump in or that falls in when you are filling up. Modern fuel has alcohol in it, and that absorbs water. Your car will "burn" any water you introduce to the fuel tank within a few tankfuls. Unless you manage to pour in more than a cupful.

    As far as accuracy, keep a logbook in the car. I record the odometer reading, distance since last filled, fuel pumped in, and calculated mileage based on those data, along with the date. I fill to the same "level" each time. I cheat, and fill till the fuel is at the top of the filler tube. This is fairly safe here because ambient temp. is almost always lower than the temp. of the fuel in the underground fuel tank at the station. So it shrinks once in the car's tank. It gives me about 200 km extra range too, though I really don't need that.

    Once you have a page of this data you have something to compare the "distance to empty" readout to. You will then have a chance to be more certain of how far you can go. And you can be more critical of the cars accuracy. Just keep in mind, it is NOT a "lab instrument". It shows you an estimate based on what it has measured recently.
     
  8. Mendel Leisk

    Mendel Leisk Sand Pounder

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    Same here. Even in a gas price war, the differences are peanuts. I don't even look at the $'s, as I'm pumping, no point. All is vanity, lol.
     
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  9. DoubleDAZ

    DoubleDAZ Senior Member

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    Are you the person I see filling up at a station even though the price at the station across the street is 10-20 cents per gallon less? :)

    As far as vanity goes, maybe to meet some arbitrary mpg mark, but for being price-conscious? :) I don't go out of my way, but if Gas Buddy tells me gas is cheaper along my route, why would I stop early and pay more just because the gauge hits the 1/2 or 1/4 point? For us, that would be throwing away $150 or more every year (I tracked for a year after finding Gas Buddy ;)). It's similar to those who brag about not using credit cards. For us, that's throwing away $600 or more every year. Using credit cards doesn't mean one has to carry a balance or pay interest. We also make 15 debits each month because that nets us $250 or so per year in interest on our checking balance instead of only the 1-3% cash-back on those purchases. Those 3 things alone add up to a tidy $1,000/yr with little to no effort. I do admit that I laugh though when I pick one station over another because it's 2 cents cheaper and then only need 8 gallons saving a whopping 16 cents. :)
     
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  10. Mendel Leisk

    Mendel Leisk Sand Pounder

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    If there's a big difference in prices, I might get motivated. But that doesn't happen much here, all the boats tend to go up and down with the same tide. There's differences of maybe 2~3 cents per liter, downtown being higher than the suburbs typically. I do stay cognisant of that.

    We're putting in 20~25 liters every 3~4 weeks, talking about $1 a month difference. And the chips fall in my favour as often as not; it's like buying mutual funds at regular intervals.
     
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  11. Redpoint5

    Redpoint5 Senior Member

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    I'm exactly like your whole post. Every little bit helps. There is a Shell station across from an Acro that regularly sells gas for $0.40 more per gallon. I wonder why anyone would go there considering Arco now has the same Top Tier fuel.
     
  12. lar.smith42

    lar.smith42 Active Member

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    this might be worth reading again from another thread.
    His flawed car had been giving him hints.

    The entire time you drive the car, it indicates how much gas you have. (My v has a count down mileage indicator until you will be out of gas. I have never ridden in a c, it may be different)
    In my Gen 2 the last pip blinked when you had better not pass a pump. (7 to 50 miles, the Gen 2 has a variable sized gas tank)
    In my v, the gas empty light lights up with 10 miles until reserve.
    In my Gen 2 when you run out of gas, every light on the dash illuminates, there is a loud beep and you can't go over 35 MPH.
    I learned my lesson with the Gen 2 and have no idea what the v actually does when you run out of gas.
    In the gen 2, you need to pull over while you still have enough HV Battery to restart the car.
    Once you get at least 3 gallons of gas in the Gen 2 (I have heard rumors that other Prius have lowered this to 2 gallons, YMMV) You can attempt to restart it without immobilizing it. It may take a restart, or two.

    No car performs well when run out of gas.

    Gasoline powered cars assume that the fuel is a liquid, once you are pumping vapors, you risk running lean and damaging the combustion chamber parts.
    Air/Fuel Ratio tuning:Rich vs Lean | Turbobygarrett

    Any modern fuel injected car (say newer than 1990) has the fuel pump in the tank, cooled by fuel, so running dry risk fuel pumpfailure.
    Tank on Empty: Burned out the fuel pump

    Now for some Prius (but not necessarily Prius c, I have not owned one) failures.
    If the car was intent on saving the car, when you ran out of gas it would go to N instantly and turn itself off once it came to a stop, this would even emulate 'normal' car behavior. Instead, the car sacrifices itself to protect the driver.

    (Pip owners may be able ignore this) Normally the EV mode is just a suggestion, any time the computers feel that too much load is put on the HV Battery, it can start the engine and fall out of EV mode. Once you have run out of gas, this protection for the battery is lost. Besides peak loads, the engine also starts when the battery gets anywhere near not being able to restart the engine. Without gas, this protection is lost.

    Now, the computers worry about draining the HV Battery when you have no fuel, (Not even the dealer has a jumpstarter for the HV Battery) so if you try repeatedly (3 times?) to restart the car with no gas, it sets a code to prevent further attempts. Get gas BEFORE you attempt to restart.

    So far as I am aware, those are the physical dangers to running out of gas. (social dangers like folks in a chat room dissing you or your wife bringing it up years later are outside the scope of my post)


    If you run the Prius dry, and take it up on it's offer to EV mode a bit further down the road, it might cost you a traction battery?
    I do not think so. If the HV Battery gets too low, it cannot restart the engine. Even the dealer has no battery charger for the HV Battery, so they have to borrow the shared one Toyota Regional has.

    So I think you are 'only' out a tow to the Toyota dealer, a week wait at the dealer for the charger and a bucket of cash.
     
  13. DoubleDAZ

    DoubleDAZ Senior Member

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    If it's like that up there, I wouldn't concern myself with prices either. It's kind of like my home state of Wisconsin where prices are quasi-regulated. When I've visited up there I only bought fuel a couple of times, so I didn't notice. One thing that was strange in Listowel ON back in '09 was that I couldn't buy gas with the same credit card more than twice a week at one service station in town.
     
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  14. CR94

    CR94 Senior Member

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    Maybe so, if you live in Edmonton. For most of us, not so, except in winter. What if fuel in that underground tank happens to be fresh off the truck, and colder than the soil temperature?
     
  15. Mendel Leisk

    Mendel Leisk Sand Pounder

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    We went to a couple of the Hawaiian islands a few years back. On Maui, no problems tanking up. But on Oahu, the pump asks you for your zip code. Erhh, that's not happening. Then it instructs you to go in, talk to the attendant. Apparently anyone out-of-state has to entrust their credit card to the attendant, tell them what pump they're on, complete manual rigmarole. What are the odds of someone out-of-state coming to Oahu? :whistle:
     
  16. lar.smith42

    lar.smith42 Active Member

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    Here almost every time I get gas the pump will ask for Zip code. That keeps somebody who has stolen my card from using it.
     
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  17. energyandair

    energyandair Active Member

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    Here is a solution that we used before we had US credit cards. Finally! Canadians able to pay at U.S. pumps with credit again - The Globe and Mail

    The article mentions Mastercard but we used it with Canadian Visa.
     
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  18. Redpoint5

    Redpoint5 Senior Member

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    The locals in Kauai don't use stolen credit cards to purchase gas; they take it directly from the fuel tank leaving you with fumes to get to the nearest station.
     
  19. MikeDee

    MikeDee Senior Member

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    Related, but a couple of weeks ago my gas light came on, got gas about 10 miles later, and it took 9.5 gallons to fill it up.
     
  20. BruceInOKC

    BruceInOKC Member

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    Yes, when it beeps and the pip flashes, it takes 9.5 gallons to fill up. At 10 gallons, it's dangerously close to running out of gas.
     
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