Consumption Screen LYING re: Mileage ?

[pola]

Yes, I've read the page to beginner's fuel economy etc.

We just took a trip in my boyfriend's '07 Prius.. He has driven 5561 miles. The consumption display screen indicated an average mileage of 41.5 mpg since last fill up. (he was using the A/C, but that is NOT the issue we are concerned about) We understand that using the A/C will decrease MPG.

What concerns us is : Calculating the actual mileage ourselves : miles driven, divided by gallons bought (from Shell) yielded a mileage of only 35.6 mpg !!!! (not 41.5 mpg, as shown on the screen)
IT APPEARS THAT THE CONSUMPTION DISPLAY SCREEN OVERSTATES THE ACTUAL MILEAGE BY 15%. !!!! ????
So when the consumption screen SAYS you are getting 47 mpg, you are really getting only 40 mpg. !!!!!
We figured this out by calculating the mpg, ourselves,... using the odometer readings at fill up time... (not going by the consumption display screen.) This was hard to believe, but true. There is no way the odometer reading was lying...
Has anyone else noticed this ?
What is going on ???

Thanks,
pola

[efusco]

Couple things...
1) Are you hitting the reset button with each fill up...if not then you're looking at a cummulative MPG for the life of the car...or at least since the last fill up.
2) The gas tank has a bladder and a quirky fuel flow shut-off that can vary widely from fill-up to fill-up. More than likely your next fill up will record a calculated MPG higher than what's displayed on the mfd. You adapt to this variability and can attenuate it somewhat by trying to use the same gas pump whenever practical for more consistant fill ups. But colder or hotter weather affect the bladder's pliability too so even then you'll have some weirdness.

3) Most of us find that over multiple tanks that the MFD overestimates FE by 1-2%, but single tanks can have dramatic variances...for a single tank the MFD is the most reliable number, over time the calculated is more accurate.

[Tideland Prius]

My mileage has been fairly close although I have had the calc'd mileage much lower than MFD on several occasions. This usually happens when you pour in more (e.g. topping off) than you used.

Check this tank. You should be able to go farther (assuming the same driving and mpg on the screen) than the past tank because you have more fuel in the tank right now.

Also, average the two tanks (both calc'd and MFD) and see what you get.

[pola]

<div class='quotetop'>QUOTE(efusco @ Oct 16 2007, 09:51 PM) [snapback]526654[/snapback]</div>

Couple things...
1) Are you hitting the reset button with each fill up...if not then you're looking at a cummulative MPG for the life of the car...or at least since the last fill up.
2) The gas tank has a bladder and a quirky fuel flow shut-off that can vary widely from fill-up to fill-up. More than likely your next fill up will record a calculated MPG higher than what's displayed on the mfd. You adapt to this variability and can attenuate it somewhat by trying to use the same gas pump whenever practical for more consistant fill ups. But colder or hotter weather affect the bladder's pliability too so even then you'll have some weirdness.

3) Most of us find that over multiple tanks that the MFD overestimates FE by 1-2%, but single tanks can have dramatic variances...for a single tank the MFD is the most reliable number, over time the calculated is more accurate.
[/b]

<div class='quotetop'>QUOTE(efusco @ Oct 16 2007, 09:51 PM) [snapback]526654[/snapback]</div>

Couple things...
1) Are you hitting the reset button with each fill up...if not then you're looking at a cummulative MPG for the life of the car...or at least since the last fill up.
2) The gas tank has a bladder and a quirky fuel flow shut-off that can vary widely from fill-up to fill-up. More than likely your next fill up will record a calculated MPG higher than what's displayed on the mfd. You adapt to this variability and can attenuate it somewhat by trying to use the same gas pump whenever practical for more consistant fill ups. But colder or hotter weather affect the bladder's pliability too so even then you'll have some weirdness.

3) Most of us find that over multiple tanks that the MFD overestimates FE by 1-2%, but single tanks can have dramatic variances...for a single tank the MFD is the most reliable number, over time the calculated is more accurate.
[/b]

THANK YOU, will send your reply to my boyfriend !!!



<div class='quotetop'>QUOTE(Tideland Prius @ Oct 16 2007, 11:06 PM) [snapback]526685[/snapback]</div>

My mileage has been fairly close although I have had the calc'd mileage much lower than MFD on several occasions. This usually happens when you pour in more (e.g. topping off) than you used.

Check this tank. You should be able to go farther (assuming the same driving and mpg on the screen) than the past tank because you have more fuel in the tank right now.

Also, average the two tanks (both calc'd and MFD) and see what you get.
[/b]

THANK YOU - will send your reply to my boyfriend !!!!

[JimboJones]

<div class='quotetop'>QUOTE(pola @ Oct 16 2007, 10:35 PM) [snapback]526650[/snapback]</div>

Thanks,
pola
[/b]

If you really want to get a tank-by-tank blow of your mpg, try to fill up when the outside air temperature indicator reads within plus minus 5 F of your last fill-up - this will take the bladder volume variance out of the equation.

Then calculate the miles you did on the amount of gasoline you filled up, and compare with the MFD.

My calculations compare quite well with the MFD.

[pola]

THANK YOU, will pass this on to boyfriend !!!!

[donee]

Hi Pola,

Last fillup MFD 58.3, by pumped gas volume 62.8 mpg. Fillup before MFD 58.4, by pumped gas volume 53.2 mpg. Filling up with the same pump both times. Yep it does vary. In the spring as the bladder is stretching out, its common to have a string of 3 or 4 tanks that give mileages well below the MFD, and this time of year the opposite. Although, this year our temperatures have been cool to hot to cool to warm. So its all over. During the middle of the summer for about a month the MFD and Pump mileage agree closely for me. I fillup at the same time of day, about 5 pm, each time. When I first got the car, I noticed a fillup in a 30 degree morning, was about 1 gallon less than at a 50 degree afternoon in the spring. Allot of plastics have a molecular phase change near freezing (+/- 20 C). In elastomers, that means less stretch.

[judymcfarland]

I'm certainly no expert, but here's how I look at it. Fill up whenever you're at 2 pips or less & over time displayed mpg will average out to something you can brag about (and I do!)

[duanelaugh]

<div class='quotetop'>QUOTE(pola @ Oct 16 2007, 10:35 PM) [snapback]526650[/snapback]</div>

Yes, I've read the page to beginner's fuel economy etc.

We just took a trip in my boyfriend's '07 Prius.. He has driven 5561 miles. The consumption display screen indicated an average mileage of 41.5 mpg since last fill up. (he was using the A/C, but that is NOT the issue we are concerned about) We understand that using the A/C will decrease MPG.

What concerns us is : Calculating the actual mileage ourselves : miles driven, divided by gallons bought (from Shell) yielded a mileage of only 35.6 mpg !!!! (not 41.5 mpg, as shown on the screen)
IT APPEARS THAT THE CONSUMPTION DISPLAY SCREEN OVERSTATES THE ACTUAL MILEAGE BY 15%. !!!! ????
So when the consumption screen SAYS you are getting 47 mpg, you are really getting only 40 mpg. !!!!!
We figured this out by calculating the mpg, ourselves,... using the odometer readings at fill up time... (not going by the consumption display screen.) This was hard to believe, but true. There is no way the odometer reading was lying...
Has anyone else noticed this ?
What is going on ???

Thanks,
pola
[/b]

After one year and 18,000 miles, I find zero error btw mfd and calculated mpg.

[pola]

<div class='quotetop'>QUOTE(donee @ Oct 17 2007, 06:03 AM) [snapback]526766[/snapback]</div>

Hi Pola,

Last fillup MFD 58.3, by pumped gas volume 62.8 mpg. Fillup before MFD 58.4, by pumped gas volume 53.2 mpg. Filling up with the same pump both times. Yep it does vary. In the spring as the bladder is stretching out, its common to have a string of 3 or 4 tanks that give mileages well below the MFD, and this time of year the opposite. Although, this year our temperatures have been cool to hot to cool to warm. So its all over. During the middle of the summer for about a month the MFD and Pump mileage agree closely for me. I fillup at the same time of day, about 5 pm, each time. When I first got the car, I noticed a fillup in a 30 degree morning, was about 1 gallon less than at a 50 degree afternoon in the spring. Allot of plastics have a molecular phase change near freezing (+/- 20 C). In elastomers, that means less stretch.
[/b]

<div class='quotetop'>QUOTE(donee @ Oct 17 2007, 06:03 AM) [snapback]526766[/snapback]</div>

Hi Pola,

Last fillup MFD 58.3, by pumped gas volume 62.8 mpg. Fillup before MFD 58.4, by pumped gas volume 53.2 mpg. Filling up with the same pump both times. Yep it does vary. In the spring as the bladder is stretching out, its common to have a string of 3 or 4 tanks that give mileages well below the MFD, and this time of year the opposite. Although, this year our temperatures have been cool to hot to cool to warm. So its all over. During the middle of the summer for about a month the MFD and Pump mileage agree closely for me. I fillup at the same time of day, about 5 pm, each time. When I first got the car, I noticed a fillup in a 30 degree morning, was about 1 gallon less than at a 50 degree afternoon in the spring. Allot of plastics have a molecular phase change near freezing (+/- 20 C). In elastomers, that means less stretch.
[/b]

THANK YOU FOR THIS ! WILL FORWARD TO BOYFRIEND and he will explain it to me ! LOL.
pola

<div class='quotetop'>QUOTE(donee @ Oct 17 2007, 06:03 AM) [snapback]526766[/snapback]</div>

Hi Pola,

Last fillup MFD 58.3, by pumped gas volume 62.8 mpg. Fillup before MFD 58.4, by pumped gas volume 53.2 mpg. Filling up with the same pump both times. Yep it does vary. In the spring as the bladder is stretching out, its common to have a string of 3 or 4 tanks that give mileages well below the MFD, and this time of year the opposite. Although, this year our temperatures have been cool to hot to cool to warm. So its all over. During the middle of the summer for about a month the MFD and Pump mileage agree closely for me. I fillup at the same time of day, about 5 pm, each time. When I first got the car, I noticed a fillup in a 30 degree morning, was about 1 gallon less than at a 50 degree afternoon in the spring. Allot of plastics have a molecular phase change near freezing (+/- 20 C). In elastomers, that means less stretch.
[/b]

THANK YOU ! !! VERY HELPFUL INFO !!!!
Pola