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This is good, but I believe that it's the MFD mpgs that count. There's too much variation in manual calculations, which are needless in this car. I consistently, tank after tank, get 50-51 mpgs on the MFD.
 

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No, no. You must calculate your gas mileage mathematically. If you're recording your MFD figures into greenhybrid's database, you are NOT using the most accurate figures possible.

Even with the variable bladder of the Prius, you will still be very accurate when calculating over the course of multiple fill-ups.

The MFD figure is only an ESTIMATION. Whereas the mathematically-calculated number is HOW FAR you drove on HOW MUCH gas.

That's gas mileage, folks. Maybe not accurate for one single tank, but as I said, from tank-to-tank-to-tank, it equalizes out, and is in fact, extremely accurate.

Mr. Mage! Congratulations, my friend! And it's good to see that you made it through the storm too!
 

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Well, I just filled my tank for the second time, and the MFD was within 1/10th of matching my calculation. (45.8mpg vs. 45.87 by my math, 9.287 gallons used for 426 miles.)

I'm hoping to pass 50mpg for a full tank soon. Once I've filled up for a third time, I'll put my stats online somewhere.
 

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BIF said:
No, no. You must calculate your gas mileage mathematically. If you're recording your MFD figures into greenhybrid's database, you are NOT using the most accurate figures possible.

Even with the variable bladder of the Prius, you will still be very accurate when calculating over the course of multiple fill-ups.

The MFD figure is only an ESTIMATION. Whereas the mathematically-calculated number is HOW FAR you drove on HOW MUCH gas.

That's gas mileage, folks. Maybe not accurate for one single tank, but as I said, from tank-to-tank-to-tank, it equalizes out, and is in fact, extremely accurate.

Mr. Mage! Congratulations, my friend! And it's good to see that you made it through the storm too!
I would dispute that. The place I get my gas has not had the pump inspected in 3 and 3/4 years!! You are assuming that what you pay for is what you get, that may not be true. A regional TV report on gas pumps reviled more than half to be off and most in the favor of the station. Washington State has 3 inspectors according to to the Tacoma paper. Can you trust your pump. Some states are good some are bad. What are you going to do!!?
 

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Half the time, I forget to write down what the pump says. The car's computer will have to suffice for me. Of course, I forget to write that down, too, but I can remember my winter worst and my summer best. To be honest, I really don't care what each individual tank is, as the variations are due to things I can't control (weather) or things I don't want to make dependent on what my car's mpg will be (how often I take the 15-minute trip to the shipping district, vs. how often I take my normal 4-mile commute downtown.)
 

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For what it's worth, I compared MFD mileage against manual calculations for every other tankful between November and April. The difference was less than 1 MPG each time (sometimes plus, sometimes minus), so I stopped checking on a regular basis.
Drive happy,
Moo :)
 

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hdrygas said:
...The place I get my gas has not had the pump inspected in 3 and 3/4 years!! You are assuming that what you pay for is what you get, that may not be true. A regional TV report on gas pumps reviled more than half to be off and most in the favor of the station. Washington State has 3 inspectors according to to the Tacoma paper. Can you trust your pump. Some states are good some are bad. What are you going to do!!?
Jamarimut's response seemed to be an attempt to deflate Mr. Mage's enthusiasm at getting great gas mileage by taking a shot at the accuracy of the variable bladder gas tank. The gas tank doesn't shrink when the temperature is above 40f, and it's Florida in August. Given this information, I think Mr. Mage's calculations are more accurate than the MFD.

Note also that Jamarimut did not mention inaccurate pumps at the gas stations as part of his reasoning.

And of course, I cannot read minds, therefore I responded to his claim that the MFD is more accurate than mathematical calculation.

I live in Mr. Mage's town. Our state and town have good enforcement with regards to weights and measures. Owners/operators of inaccurate pumps could be prosecuted as fraudulent in the state of Florida.

If you have a fraud problem in your state, then there's little that can be done to insure accurate mileage calculations. In fact, you have a more serious problem on your hands, and I would urge you to complain to the appropriate authorities. Or sue. Surely you can find a lawyer to take the case as a class action in the interest of the public in your state. There may be a shortage of inspectors, but the last I heard, there was no shortage of lawyers. :? Remember too, if this type of fraud is allowed to continue, then there's no telling what other types of fraud are happening.

If it's a losing battle no matter what you do, then maybe other things are more important to the people in your state. After all, we as a society usually get what we want.

In any case, I stand by my original statement. The MFD is only an estimate, using the amount and and duration of fuel injector flow. Personally, I have wild fluctuations between MFD and calculated mileage whenever I burn less than a half-tank. The differences are slimmer when I burn nearly the whole tank.
 

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If you consistently drive at, say, 50mpg the whole tank they will be a textbook match. But if you drive so that the Average MPG's change all the time, then they will be different.
 

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BIF said:
The MFD figure is only an ESTIMATION. Whereas the mathematically-calculated number is HOW FAR you drove on HOW MUCH gas.
The MFD figure is NOT an estimation. It is a very thorough calculation of fuel used in millisecond intervals. There is no estimating done. The calculation uses the actual injector on time for each fuel injection period, injector flow rate, and distance traveled to arrive at the calculation. The accuracy is within 0.1 mile per gallon for any amount of fuel used greater than 1 gallon. Less than 1 gallon the accuracy is limited by display resolution, offering up to 10% error. The MFD reading you see is within 2%, 1% typical, of the actual fuel economy achieved. It will take at least 20 fill-ups using manual calculations to begin to average out the affects of fuel fill variability. Even then, you will have up to 1 gallon of fill variation tank to tank as a result of the bladder and pump shut off sensitivity.
 

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Even without bladder variations (i.e., constant temperature & after any necessary break-in period) I am inclined to believe that the fill variation is likely to be greater than the car's mpg error. I see a much more consistent tank-to-tank reading on the computer than I get by calculating. My calculated numbers (for those tanks I remember to write down) jump way up and down, while the displayed mpg has risen steadily from the dead of winter to mid-summer.

I don't look forward to the return of winter, but it will be interesting to see if my now broken-in car does better this winter than it did brand-new last winter.
 

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This weekend I drove a 300 mile round trip from central Connecticut to southern New Hampshire. I drove up to New Hampshire on I-91 and I drove back Rt. 10 to Northampton before I got on I-91, as a result I averaged 56 mpg round trip. The best mileage yet in the car! If I had taken the highway back I would have averaged about 52-53 mpg.
I used cruse control most of the time set to about 5 mph above the speed limit
 

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MFD is very accurate

It is very easy for you all to demonstrate to yourselves that trying to
calculate gas mileage on a Prius from the pump data is difficult unless you fill up at the same pump each time.

When you log data, also log the MFD reading with your odometer and fillup amount. Divide the number of miles you drove by the MFD reading. This is the amount you would predict your car to take at the pump. If you fill up at the same pump each time you will see less than 1 cup variance in the amount of gas the pump will put into your car when it AUTOMATICALLY shuts off. After doing this a few times, change gas stations. You may see a large change in where the pump shuts off. I have seen as much as 5 quart difference between pumps. This gives a significant error in your fuel economy calculation. I have attached a pdf of my spreadsheet showing this. The fill ups with the least error where at the same pump. Notice how closely they match the MFD.
 

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...but from tank to tank to tank to tank, it doesn't matter so much whether you fill up at the same pump.

Because over time, you will still be recording how far you drove on how much gas. And somebody mentioned 20 tankfulls. I don't know how the magic number became 20...that doesn't make sense. I think even three tankfulls, particularly if you're filling up more than half the tank each time, would be pretty accurate. And nobody stops driving their car after three tankfulls. We keep driving. The overall margin of error will get less with each new tankful.


And when all is said and done, some of us just want to measure our mileage. So leave us be, let us measure our mileage, and let us get excited and enthusiastic when we have a good tank! After all, somebody else's good measurements aren't hurting you, are they? :?
 

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I think I really got hosed on gallon measurement in Ellensburg WA, on my way home. 44.3 mpg calculated. 49.8 showing on the computer. All highway miles on I-90 starting temp mid 60's, high temp mid low 90's little or no wind. Paid $.12 a gallon more than home. Suspect that this place in a highway rip-off spot in more ways than one. First tank no way to check dealer fill, am on my third. Anyone else seen such a large discrepancy?
 

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Yes, hdrygas, I also had a pretty big discrepancy on my first tankful and it has been going down over time. Furthermore, I seem to recall that the gas gauge moved off the fill line sooner on the first tankful than on subsequent tankfuls, so I suspect the dealer didn't fill it quite all the way.

After my first fillup (8 gallons), I computed 44.3 mpg while the screen read 50.7. However, after 4 fillups (~36 gallons), the discrepancy in lifetime MPG is down to 48.4 by calculation of total miles / total gallons vs. 50.3 by screen.

[Note that getting your lifetime calculation of MPG computed via the screen is a little subtle...Each time the screen MPG is reset (e.g, at fill-up), you must divide the miles traveled shown on the screen by the MPG shown on the screen to compute the gallons of gas used via the screen calculation. Then to get lifetime MPG average as computed by the screen, you add up all the gallons of gas used via screen calculation and then you need to divide the total odometer mileage by this sum. I created a little Excel spreadsheet to do this.]
 

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Discussion Starter #18
jshore said:
I also had a pretty big discrepancy on my first tankful and it has been going down over time.
Same here; then it went back up. :p However, with the gradual/overall increase in per tank mpg, my calculated lifetime mpg has also been going up. Of course, I still have a LONG way to go to catch up to BIF, but I'm still learnin'. 8)
 

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I don't trust the pumps in Washington because they are so rarely inspected. The one I use on my way home has not been inspected in 3 years next month. I read in the paper there are just three inspectors in the whole state.
 

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BIF said:
And when all is said and done, some of us just want to measure our mileage. So leave us be, let us measure our mileage, and let us get excited and enthusiastic when we have a good tank! After all, somebody else's good measurements aren't hurting you, are they?" :?
Whats up with this statement?

Are you saying that we should not post information about gas mileage? Are you saying that if you get a sudden drop in mileage because of low tire pressure or some mechnaical problem that it should be ignored as long as the lifetime mileage is OK? That could be a long time if you have a lot of miles on the car.

No one is trying to get you to do anything, but if people are presenting false mileage info based on bad math, then YES it hurts the Prius community’s credibility.
 
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