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Discussion Starter · #1 ·
Do most of the prius owners use the MPG readouts on the prius to calculate mileage or do it themselves with the odometer?

I ask, because it occured to me that it might be possible to increase mpg by never filling the talk over half way. With people getting over 600 miles per tank, I think a range of 300 miles would suit me fine.

For the record, I belive a half tank (5.95 US Gal) of gas weights about <correction> 36lbs </correction>

If you use the Prius MPG display to calculate, how do you (I don't own a prius yet, so I'm curious).

-Dan <11011011>
 

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Amputating your left leg will improve mileage by about the same amount.

Some of you will say that seems extreme, but do you REALLY need that leg to operate the Prius?
 

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Oh, actual on-topic reply: I use a database program (Filemaker) to calculate lifetime mpg, using both pump and MFD calculations. After seven months, the pump calculation is a fairly steady 2 mpg lower than the screen. (Which could mean nothing more than that Colorado is home to lots of crooked gas station operators!)
 

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Discussion Starter · #6 ·
What about the calculations

I've also read that the bladder in the prius will fill to diffenent amounts depending on the temperature "13% variance".

So is everyone using the prius's internal mpg calculator to calculate thier mileage?

-Dan <11011011>
http://ScreamingMonkeys.blogspot.com
 

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Re: What about the calculations

Dan. said:
I've also read that the bladder in the prius will fill to diffenent amounts depending on the temperature "13% variance".
Wait until winter. You won't be reading it, you'll be LIVING it!
Dan. said:
So is everyone using the prius's internal mpg calculator to calculate thier mileage?
No. See previous post.
 

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Re: What about the calculations

Dan. said:
I've also read that the bladder in the prius will fill to diffenent amounts depending on the temperature "13% variance".

So is everyone using the prius's internal mpg calculator to calculate thier mileage?

-Dan <11011011>
http://ScreamingMonkeys.blogspot.com
DUMB question time, but here goes: What internal MPG calculator? Or that where it says 'average' MPG on one screen? Because, if that is what people use to figure the mielage, then I would guess their mileage is way off from the ODO.

I could be wrong.
 

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Re: What about the calculations

TJandGENESIS said:
Dan. said:
I've also read that the bladder in the prius will fill to diffenent amounts depending on the temperature "13% variance".

So is everyone using the prius's internal mpg calculator to calculate thier mileage?

-Dan <11011011>
http://ScreamingMonkeys.blogspot.com
DUMB question time, but here goes: What internal MPG calculator? Or that where it says 'average' MPG on one screen? Because, if that is what people use to figure the mielage, then I would guess their mileage is way off from the ODO.

I could be wrong.
Yes, the screen. It's actually very close to the odometer/pump calculation, though most drivers find it to be a little optimistic.

As for which is actually more accurate, nobody knows. Any calculation from the odometer/gas pump is only as accurate as the gas pump, which apparently is not legally required to be all that accurate (despite the three decimal places on new pumps). We're used to assuming the pump is accurate because until now we didn't have any other way of calculating mileage.
 

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Discussion Starter · #10 ·
Seems hard to konw for sure what it is in the end.

coloradospringsprius said:
Oh, actual on-topic reply: I use a database program (Filemaker) to calculate lifetime mpg, using both pump and MFD calculations. After seven months, the pump calculation is a fairly steady 2 mpg lower than the screen. (Which could mean nothing more than that Colorado is home to lots of crooked gas station operators!)
I guess my question is if the automatic mileage calculator is optimistic, and the size of the gas tank varies drasticly with fluctuations in the weather, how can you possibly calculate the amount of gas you burn, short of weighing the car?

Example... Lets say I buy my car and it's 95oF outside, top off the tank (with a warm weather capcity of 11.9 gal), and reset the ODO. 500 miles later (which by my driving will be about 3-4 weeks) the temperature is 40oF (look at an alminac for Texas, it's common). I look at the pump and notice I put 9 gallons in the car. If the blader shrank to 10.8 gal becuase of the weather, I'd never know it. So going on ODO/pump , 500/9 = 55mpg. But since the capacity of the tank shrank a gallon since I last filled up, the acurate calculation would be 500/10 = 50mpg. That is a huge varience.

I'll concede that these fluctuations should go both directions and in the end the numbers should converge, but it seems a real guestimation whenever anyone runs thier mileage....

But like I said earlier, I don't own one yet, so don't really know whats what.

In the end sounds like most use ODO/pump...

Thanks
 

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I stopped worrying about it the day I finally got fed up with my pocket pc hard-resetting itself at inconvenient times...

However, up until that time, I kept careful track of both on my speadsheet which you can find here: http://home.cfl.rr.com/njfiorello/priuslog.xls
 

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I'm just shy of 5000 miles and my experience has been that the MFD is more accurate than my calculated MPG. I believe this is because of erratic fill ups. The biggest deviations have been at unfamiliar gas stations. Some pumps just seem to have overly sensitive auto-shutoffs. I generally refill at 2 bars on the gage, just over 400 miles, so I can use the same station near my home and minimize that variation. I also never top off as I believe that partially defeats the effectiveness of the evaporative emmisions system. Although the average difference between indicated and calculated MPG is +/- 2 MPG, my lifetime indicated avg. is 59.32 MPG and my lifetime calculated avg. is 58.66 so it seems to even out.
 

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Re: Seems hard to konw for sure what it is in the end.

Dan. said:
coloradospringsprius said:
Oh, actual on-topic reply: I use a database program (Filemaker) to calculate lifetime mpg, using both pump and MFD calculations. After seven months, the pump calculation is a fairly steady 2 mpg lower than the screen. (Which could mean nothing more than that Colorado is home to lots of crooked gas station operators!)
I guess my question is if the automatic mileage calculator is optimistic, and the size of the gas tank varies drasticly with fluctuations in the weather, how can you possibly calculate the amount of gas you burn, short of weighing the car?

Example... Lets say I buy my car and it's 95oF outside, top off the tank (with a warm weather capcity of 11.9 gal), and reset the ODO. 500 miles later (which by my driving will be about 3-4 weeks) the temperature is 40oF (look at an alminac for Texas, it's common). I look at the pump and notice I put 9 gallons in the car. If the blader shrank to 10.8 gal becuase of the weather, I'd never know it. So going on ODO/pump , 500/9 = 55mpg. But since the capacity of the tank shrank a gallon since I last filled up, the acurate calculation would be 500/10 = 50mpg. That is a huge varience.

I'll concede that these fluctuations should go both directions and in the end the numbers should converge, but it seems a real guestimation whenever anyone runs thier mileage....

But like I said earlier, I don't own one yet, so don't really know whats what.

In the end sounds like most use ODO/pump...

Thanks
The Prius' gas tank is actually even quirkier than you've heard: In addition to the changing bladder size, it's notoriously difficult to get consistent fill levels. Because of this, for any individual tank, the screen MPG is the most accurate information available about your gas mileage, period, end of report. However, over the course of several tanks, the ODO/pump calculation may be more accurate.

There's only one reason why I use the ODO/pump calculation when talking about mpg. It may or may not be accurate, but it's based on the amount of gasoline I PAID for.
 

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Thanks for the replies. As I once said to a friend, I'm happy that I am getting more then 30 MPG.

And I stil am. 45, 50, 60...doesn't matter, as long as it's over 30.


Maybe I have low expectations, but what the heck. I'm happy.
 

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Dan, your logic is a little flawed. You don't calculate mileage based on what percentage of the tank you filled, but on the actual gallons pumped. It doesn't matter if the Prius has a 50 gallon tank, pumping in 9 gallons is still pumping in 9 gallons.

I use a spreadsheet to keep track of both MFD mileage (car-computer-calculated that resets every tank,) and pumped mileage (miles driven divided by gallons pumped.) Right now they're off by 2.5%. Calculating by gas pumped gives me a lifetime MPG of 45.08, calculating by what the computer has said at the end of every tank gives me 46.22. (As of my last fill.)

And, hey, I just noticed that I have pumped exactly and precisely 500 gallons of gas into my car! (At an average cost of 4.7 cents per mile in fuel costs.) The mileage figure given in my avatar is based on car-computer calculated. (I like to be optimistic.) It's also based on how often I actually update it. I think it's four or five days old now, I'll have to update it. (Current as of the writing of this message is 419 days, 22761 miles, 46.28 miles per gallon.)
 

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ehurtley said:
Dan, your logic is a little flawed. You don't calculate mileage based on what percentage of the tank you filled, but on the actual gallons pumped. It doesn't matter if the Prius has a 50 gallon tank, pumping in 9 gallons is still pumping in 9 gallons.
I think what Dan is getting at though, is that if he doesn't fill pass the half way mark such that his 9 gallons is split up into 2 visits to the gas station, then he doesn't have to carry that second 4.5 gallons until he is ready to use it. The savings in weight lets him go farther with his first 4.5 gallons than than if he bought all 9 gallons at once on the first visit. Thus, a farther distance traveled with the same amount of gas means a higher MPG.

Louie
 

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The screen's MPG does seem to increase as I near the end of the tank. One would think that this would be because of the reduced weight. I don't think the weight issue is significant enough to bring the MPG up that high.

Here's my other theory: Fuel vapor. The screen MPG is calculated by injector on time. If the intake manifold were to get fuel from some other source, then the O2 sensor would tell the ECM that the fuel is too rich, and so the ECM would cut back on the injector ON time, thus making it seem you are using less fuel. At least when you start the car, and possibly at other times during a trip, a vacuume is applied to the tank for two purposes: to recapture and burn fuel vapor, and check for vapor leaks in the tank. The air that is sucked out of the tank is dumped into the intake manifold where it is burned. Towards the end of the tank, even with the bladder there still is a void partly filled with fuel vapor.

From the perspective of one tank, using the MFD MPG would be more accurate and precise, as you can't be sure if you filled to the same level you started your measurements. But, if you combine several tanks, then the fill level error starts to pale in comparison to the gallons used. The max error you could get is 12 gallons, as that's all the tank will hold. In practicality, the error is probably no more the 2 or 3 gallons. Over 10 tanks from full to near empty, you're talking about 100 to 120 gallons over an error of about 3 gallons, or 2.7% (3 gallon error over 110 gallons, assuming 11 gallons average per tank for 10 tanks). However the tank error is probably half that.

Now the pump error is a different matter. Only a government official would be able to check that, or you can check it using an accurate measuring apparatus before pouring into the tank.

The injectors themselves are rated for an error of 3%. The O2 and ECM compensate for this error, but they would not know the source of the error. All they do is balance the multivariable equation using injector on time as the adjusting variable. Input variables that determine initial fuel injected include air temperature and pressure, power demand, engine RPM. Engine, fuel and air quality would affect the resulting burn results, which are not directly measurable. This is why stable systems use closed loop feedback, to compensate for what can't be directly determined.
 

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Another problem with using the MFD to calculate lifetime mileage is that, when calculating the average mileage, the mileage of each tank must be multiplied by the amount of gas used in that tank, not by the distance traveled.

To use the MFD calculations to accurately determine lifetime MPG, you must calculate the gas consumed for each tank (divide MFD tank miles by MFD tank MPG) and use the total calculated gas consumed to date and odometer reading to calculate average MPG.

Assume you fill your tank for the first time at 300 miles, and the MFD says 30MPG for 300 miles. You fill up again at 600 miles, and the MFD says 50MPG for 300 miles. You average consumption for the first 600 miles is 37.5 MPG (not 40 MPG). The car burned 16 gal over the 600 miles (10 for the first 300, and 6 for the next 300).
 

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You keep forgetting the real Prius

You keep forgetting the real Prius

The real Prius is TEMPERAMENTAL.!
You will never know its MPG...!

I am tired to hear of the fricking bladder, gases, gas, tank capacity, temperature, thermos, terrain, weight, accessories working or not, driving habits, traffic conditions, electric time mode, gas mode time, odometer, fillups and half fillups calculations, etc.
You have to much variables and you have combined in a million ways to obtain just one constant value:

YOU DO NOT KNOW THE PRIUS'S MPG YET.

I can only repeat what I said: THE PRIUS IS A TEMPERAMENTAL CAR OR A COMPUTER BOX THAT NEVER SAYS THE REAL THING CONSTANTLY.

Please, do not count a lot in the electric motor to help you lifting weights or at high speeds. Remember the insufficiency I mentioned.

I just was passing by the forum. I continue working in my project of the Mechanical Torque Multiplyer.
 

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"Remember the insufficiency I mentioned."

Have no fear, insufficiency is always on our mind when we read your posts.
 
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