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post #1 of 10 (permalink) Old 07-30-2008, 06:03 PM Thread Starter
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Default Fuel tank and MPG confusion

I know the fuel tank has a bladder and can hold 11.9 gallons of fuel. I ran the tank dry and it needed to be pushed to the gas station. My doc would love the fact i am getting excersize pushing my car all over. When I went to fuel, the pump clicked off at 10.334 gallons. I went 675.2 miles and when I do the math, the computer mpg and the calculated do not add up. How does the Prius calculate MPG? It was 3.3 mpg lower than the calculated mpg. Wierd
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post #2 of 10 (permalink) Old 07-30-2008, 08:39 PM
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Default Re: Fuel tank and MPG confusion

You partially answered your own question in your first sentence. The fuel bladder is flexible and, therefore, holds varying amounts of fuel. 10.334 gallons is in the ballpark. I generally refuel as soon as the last pip starts blinking and sometimes before (hint, hint ), and in warm weather the first pump click-off generally occurs around 9-10 gallons.

Wait until winter; it will hold even less.

Because of the tank-to-tank variation, the MFD mileage likely will be more accurate than calculations for any single tank. Calculations become more accurate over multiple tanks. I have tracked the difference between the two for about ten months, with the MFD showing about 2.5% more overall than calculated.

Here's an article that describes the fuel system in depth, including (page 3) how fuel mileage is calculated:

http://www.autoshop101.com/forms/Hybrid13.pdf
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post #3 of 10 (permalink) Old 07-31-2008, 01:55 PM
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Default Re: Fuel tank and MPG confusion

Briefly, it multiplies fuel injector actuations and durations to get the fuel used and divides that by miles traveled. The fuel tank gas gauge is not involved in this calculation because it is not very accurate. The tank bladder variability means that the amount of fuel burned since the last fillup is usually not the same as the amount you put in at the current fullup, so manual MPG calculation of a single tank will usually be inaccurate regardless of the accuracy of the gas gauge.

The bottom line is, when the last pip on the fuel gauge starts to flash it means "buy gas right now, **** it". As you have learned one cannot know exactly how much gas is available (note "available") in the bladder after the last pip.

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post #4 of 10 (permalink) Old 07-31-2008, 04:44 PM Thread Starter
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Default Re: Fuel tank and MPG confusion

is there a way to make it more accurate?
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post #5 of 10 (permalink) Old 07-31-2008, 05:06 PM
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Default Re: Fuel tank and MPG confusion

If you want to re-design and re-build the tank, yes. Otherwise no.

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post #6 of 10 (permalink) Old 08-03-2008, 10:56 AM
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Default Re: Fuel tank and MPG confusion

I calculated my mileage for an entire year. By the pump, it was 46.36, by the computer it was 46.99. This makes the computer 1.4% optimistic. But, gas pumps are not perfectly accurate. So, all I can say is that the two are very close. After 10,000 miles they differed by only three gallons, $12.00, not very much in the total cost of driving the car.

I ran my tank dry once and got 10.9 gallons in. In very cold weather I will get less than seven gallons in after the last square is blinking. Also, in the winter the first square goes out after 15-50 miles, in the summer the first square goes out after 70-160 miles.

The gas gauge measures the level in the tank. In cold weather, the top of the bladder does not expand. The one thing that the gauge is quite good at is warning: when the last square is by itself, start looking for gas; when it starts blinking, get gas.
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post #7 of 10 (permalink) Old 08-03-2008, 02:39 PM Thread Starter
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Default Re: Fuel tank and MPG confusion

Quote:
I ran my tank dry once and got 10.9 gallons in. In very cold weather I will get less than seven gallons in after the last square is blinking. Also, in the winter the first square goes out after 15-50 miles, in the summer the first square goes out after 70-160 miles.

The gas gauge measures the level in the tank. In cold weather, the top of the bladder does not expand. The one thing that the gauge is quite good at is warning: when the last square is by itself, start looking for gas; when it starts blinking, get gas.
Good idea. Mine went 675 miles before it quit. I pushed for .2 miles to the station and made it. Sometimes I've gone 214 miles before the first square went out. I don't mind if i haven't reached the station on account of doctors advice, it's passing it i'd be mad about.
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post #8 of 10 (permalink) Old 08-03-2008, 04:17 PM
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Default Re: Fuel tank and MPG confusion

The computer uses the flow meter on the first cylinder fuel injector (actually on all Toyotas that have computer based mileage). It can be accurate for the amount it measures but it doesn't measure if the tank is absolutely full. It (usually) takes a reset signal from the tank when the tank measures it is full, which as has been stated before, can vary greatly.

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post #9 of 10 (permalink) Old 08-03-2008, 07:55 PM Thread Starter
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Default Re: Fuel tank and MPG confusion

most appreciated! I did not realize the fuel tank was that SOPHISTICATED!!!!! That is so sweet!!!!
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post #10 of 10 (permalink) Old 08-03-2008, 08:31 PM
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Default Re: Fuel tank and MPG confusion

Not too sure about being "sweet." I believe most owners if given the choice would "opt" for the 12 gallon tank furnished with with the Prius sold to the rest of the world without the bladder.



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