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Reserve Tank

Discussion in 'Gen 3 Prius Fuel Economy' started by TLCESQ, Nov 8, 2014.

  1. TLCESQ

    TLCESQ Member

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    Today the Service Dept told me I had a 2-gallon reserve tank, which is why I could only fill up to about 9.5 gallons when I drive into the gas station literally on empty. Anyone ever heard this?
     
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    I do not think it is a separate tank, but yes, due to the potential damage if you ever run out of gas, there is about 2 gallons left when it shows empty.

    Do your best to never use it.
     
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  3. bisco

    bisco cookie crumbler

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    a reserve tank?:confused:
     
  4. xliderider

    xliderider Senior Member

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    2 gallons of reserve in the tank. ;)

    SCH-I535
     
  5. TLCESQ

    TLCESQ Member

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    So the 2-gallon gas reserve shouldn't factor in mileage computation then?
    When I fill up from 0, it displays a range of 562. In reality (for the last 3 tankfuls I've been tracking), I'm getting 470-489. Does that make sense?
    For tracking purposes, I use Trip B to track overall mileage per tank, while Trip A is reset for each trip within that tankful. Most segments are 42 miles, averaging 54 mpg consumption. If I'm averaging 54 mpg for each segment, where's the other 73 mpg (562-489)?
     
  6. bisco

    bisco cookie crumbler

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    no. you can go farther, but i wouldn't push it, it's there for safety, in case you're not near a gas station.
     
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  7. TLCESQ

    TLCESQ Member

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    I'm just trying to get an accurate idea of MPG. Is there another way to do it that isn't too technical; I'm horrible wih math......
     
  8. fuzzy1

    fuzzy1 Senior Member

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    As Jimbo said, the reserve is not in a separate tank, but is in the main tank below the point the gauges show empty.

    This gauge 'error' is not unique to the Prius, but is the norm across the auto industry. If the manufacturers didn't intentionally do this, the combination of cheap inaccurate components, highly variable and unpredictable fuel consumption, and poor driver attention and planning, would cause far more drivers to run out of gas. And they would blame the car makers on the many occasions when the gauge earlier displayed enough fuel before MPG conditions deteriorated. So the gauges have enough built-in warning that when drivers find themselves stranded, they cannot rationally claim lack of warning.

    For a better feel of how much range you probably have left, read the first post of this long thread: [WARNING] Running out of gas (Gen III).
     
  9. bisco

    bisco cookie crumbler

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    well, it takes a little bit of math. fill the tank, and divide the miles driven by the gallons. for example, 480 miles divided by 9.5 gallons equals approximately 50 mpg. you can use a calculator in this class.:)
     
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  10. fuzzy1

    fuzzy1 Senior Member

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    Miles remaining, fuel reserve, etc., are irrelevant when trying to get an accurate idea of MPG. These are just warning to the driver to help prevent running out of fuel.

    Just keep a fuel log, showing your Trip B miles between refills, and the amount of fuel added when refilling. MPG = (Trip B miles ) / (gallons pumped in). That is all you really need.

    If you are horrible with math, than playing brinksmanship with an emptying fuel tank is going to leave you stranded by miscalculations and the inherent uncertainty in knowing how much gas is actually available, and how much is being burned as conditions vary. MPG is almost never a fixed number, it varies widely even during a single trip.
     
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  11. TLCESQ

    TLCESQ Member

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    So use 9 gallons as the capacity despite the claim by Toyota that it's an 11.9 tank size. (I wonder if this would factor into a claim like that leveled against Kia last week for MPG misrepresentation......)

    Good to know -- thanks. And the GenIII thread suggested above was helpful. Thanks for the guidance, everyone.
     
  12. bisco

    bisco cookie crumbler

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    no, it really is a 11.9 gallon tank. how much of it you use doesn't affect mpg, just range. all cars are like this, is this your first?
     
  13. xliderider

    xliderider Senior Member

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    Register with Fuelly.com and when you fill up your tank, just record the odometer miles, gallons filled, and price per gallon (get a receipt). Then enter your odometer reading in fuelly.com, it will calculate your mpg for you. ;)

    SCH-I535
     
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  14. TLCESQ

    TLCESQ Member

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    First Prius? Yes. First car -- nope. Have had many, but never cared about mpg before. Late to the game.....
     
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  15. TLCESQ

    TLCESQ Member

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    Thanks!! Will do.
     
  16. bisco

    bisco cookie crumbler

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    it's funny what you don't notice about a car when you aren't paying attention to mpg's.;)
     
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  17. TLCESQ

    TLCESQ Member

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    Thanks for this suggestion. Much easier than using Evernote.
     
  18. TLCESQ

    TLCESQ Member

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    Last 2 vehicles were Suburbans. Last thing you can want to see regularly is how fast you are burning through petrol.
     
  19. Mendel Leisk

    Mendel Leisk Senior Member

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    Forget about all the car's meters. Record your odometer reading when you fill up. Drive 'till tank is lowish. Fill up, and note new odometer reading. Subtract old odo reading from new. That's your distance travelled. Divide that number by the gallons you put in. That's your mpg.
     
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  20. Nora

    Nora Member

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    Fuel consumption or fuel economy doesn't have anything to do with size of the tank or how much fuel is in it when you fill up. In the US, we measure it as miles per gallon. (In the rest of the world, it's liters per 100 km, which is the inverse of mpg, in the "math is confusing" realm, so let's not talk about that now.) If you drive 500 miles and fill up with 10 gallons, you've got 50 mpg. If you decide for whatever reason to fill up after 250 miles, you're going to fill up the tank with something like 5 gallons, so still 50 mpg. If you decide to fill up after driving only 100 miles since your last fill up, you're only going to get 2 gallons in that tank.

    What really messes up your numbers is not filling the tank. I'm sure I'm not the only one who remembers being cash strapped during the gas crisis of the 70s and only buying $2 worth of gas. Don't do that. If you fill up the tank every time, each calculation of miles driven since the last fill up divided by gallons pumped today will give you an accurate mpg. Of course, that mpg is going to vary, but not a lot. Oh yeah, and don't run the tank dry. It's got nothing to do with fuel economy, it's just a bad idea.
     
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